Randy Calvert

This depends on how you've configured your settings.
ACP > System > Settings > Posting

If whatever time frame you have specified has not passed, you'll find it in the ModCP under "Deleted Content".

Have you confirmed your server has the mod_rewrite module enabled and working correctly? It may need enabled for your specific hostname as well as that can be turned on/off at the vhost level. The screenshot looks IPS is checking for your mod_rewrite and not seeing it properly so telling you to fix it first.

ACP > System > Site Features > Applications
You can also get to it via:
YOURDOMAIN.COM/admin/?app=core&module=applications&controller=applications
Expand the "System" section.  Under it will be a list of modules.

I'm guessing for you "Moderator CP" will show as "Disabled".  Just click the "Disabled" icon to switch it back to "Enabled". 

To be honest, it's not worth it for a month or so.  It's a very manual process on both ends and I believe if you are not planning to host for a year, the import fee is around $500.  The import was free when I moved my site as I signed up for a year.
Essentially you would agree on a date with the staff to turn your current site offline and provide a full copy of your site files along with an export of your database.  (This is done so you don't lose content from when you export and when your site is actually moved.)  You would upload everything to a space provided by IPS.  A member of the staff would import everything and make it available on a temporary URL.  Once you are happy, you would confirm with IPS and point your DNS to them. 
When I did it, I turned my site off at midnight eastern and exported everything.  Marc had everything imported for testing by 7am and I was done testing by 7:30.  By 8:00, DNS pointing was complete and the site was live.
To leave would be a similar process... you would need to turn your board offline.  Afterwards, a member of the staff would then export a copy of your database.  You would need an active self-hosted license to receive the board files, otherwise you would get just the database and the uploads/applications folder (everything but the codebase).  
 

It does not reset.  So up and up forever.  🙂 

You might need to try smaller batches.  Try much smaller until you find the number range that works for you.  It might only be 100 in a go.  
Otherwise, you could work with IPS sales and they might be able to do some sort of mass import for a fee as well.  
https://invisioncommunity.com/contact-us/

Go to your Client Area (https://invisioncommunity.com/clientarea). Find your purchase and choose the link that says "Manage This Purchase". Below the Active User count is the info about your total space used.  

It’s not the way the IPB software works. It’s always been like that. You can post a request in the feedback forum if you like for it to be considered to be changed. 

How easy this is will depend on your configuration settings.  
ACP > System > Settings > Posting

If it's set to "Immediately" or to a day prior to the 31st, the only way to recover it is to have IPB 100% restore your site to a date prior to the deletion.  Meaning you would lose all posts/activity that happened since the restore was done.  
If your settings are configured not to permanently delete content prior to that point, you can restore it from the ModCP.

You can restore it yourself.  🙂  Click the checkbox to the right and choose restore.  

It's possible to do, but that means you would lose history associated with that account.  For example, for banned members we will continue to track device IDs, emails, IP address ranges to see if a banned member simply creates a new account using a different email.  If you delete the account, any history you have on it is also lost.  

That would depend on a lot of circumstances.  When using a web interface, the problem is how long it takes in time, not how many records you have.  (It will execute for XX seconds before giving up.)  So one batch may work at 10k, but another might fail at 5k because there was something that took longer to process in that smaller batch.  
Personally if you're using something that big, it might be worth using the API to create the accounts.  
https://invisioncommunity.com/developers/rest-api?endpoint=core/members/POSTindex
If you don't want to do that, start small...  1k and see if it works.  If it does, increase it and try a bigger batch.  Rinse and repeat until you find what works for you.

So...  let's pause here for a moment to consider a few things:
This does not appear to be an IPS problem.  It appears to be the problem of a specific 3rd party resource provider.  This does not appear to be impacting a large swath of IPS customers, so this would seem to suggest this was not an exploit in the IPS software itself.   The domain does NOT appear to be hosted by IPS cloud.  It's pointed to Hivelocity.  If the domain is supposed to be hosted by IPS, that means the DNS for the domain was hijacked.  (Not an IPS issue.) The domain is not using IPS' name servers...  they're using the default Namecheap registrar name servers.  (This leads me to believe the site is self-hosted) If the domain is supposed to be self-hosted, it looks like the self-hosted web hosting account or their server/VPS, is hijacked or that the compromise happened by some other means (either weak 3rd party resources, other non-IPS software running on the site, etc.) No one except the site owner (the 3rd party resource author) can say exactly what happened with their site and if any data was compromised.  By default, things like credit card and password information is encrypted by the IPB software.   You might want to reach out to that provider directly for assistance as IPS itself cannot do anything in this case.  

When an admin sets a default language, that’s the language that will show if someone is not logged in or if the user has not implicitly selected a language. Once a user chooses a language, as long as they’re logged in, that language choice will be honored. (The same is true for themes as well.)

Google Analytics does not include bots (such as Google itself) or entities that do not download Javascript.  IPB is not reliant on downloading of javascript to track activity, so they do see/show them.  
The GA view will most likely be a more accurate view of real "humans".  

You can limit the areas of the ACP that they can access by making them a Restricted Admin.
Check out the following guide:
 

No. There is no “other admin confirmation”. They are either an admin or they’re not. 

If you want to review actions they’ve taken, there is both moderator and admin logs available to show what actions have been performed by others. 

If they are using an IP address you previously banned, then yes... they would receive that error anytime they happen to be on that address.   You should be able to look up the member in the ACP and see the IP address they used and then compare it to what is in your banned IP list.  If the IP address is there (including as part of a bigger subnet block), you would need to remove it.

What happens if you remove Sendgrid’s credential details from IPB. 

We might as well cut to the chase and skip the examples. I can give other examples of not expiring subscriptions such as Netflix or my cable bill. It does not matter ultimately. What you are wanting to do is not possible with the software. You will either need to change your model, hire a 3rd party resource provider to create the functionality, or use a different solution. 

Not found would indicate either you uploaded files to the wrong spot or your server is configured wrong.  In either case, it's not a software issue.  
A 404 error (not found) would not be a PHP issue regardless of version.  The server can't find a file to even have it processed by PHP.  

What actions do you have configured when an account is marked as a spammer?
If you are not banning them, they would still be allowed to change a profile picture. 

You most likely have multiple login methods… for example do you also have a converter installed?  Check those other methods if you could. 

Have you installed the latest patch inside of your ACP?  This was just fixed yesterday. 

POP3 is related to retrieving email.  The only place that I know that uses INBOUND email is the support section of the commerce application.
ACP > Commerce > Support > Settings
There is a tab for Incoming Email.  Select it and correct the info there.

Edit: Marc was faster than me! 😄 

I don't believe it's the forum software.  When I see this happen, it's because the browser has reclaimed memory.  It does not just happen on IPB, so if I was a bettin' man, I would say it was a function of the browser/OS and most likely a function of the browser trying to manage resource utilization.  
Take a look at:
https://support.google.com/chrome/thread/204546987/why-chrome-tabs-refresh-automatically-when-i-go-to-another-tab-and-how-do-i-stop-it?hl=en
https://workspacetips.io/tips/chrome/how-to-stop-chrome-from-refreshing-your-tabs/