Jim M

At this time, we have not fully claimed support for 8.1. Testing is still something which we have on our to-do list.

Are all sub-menu items from all navigations removed or just from "Forum"?

Are you requiring a Billing Address though?

Yes, you can certainly manually upload these contents if you so desire. When you change your storage configuration, you will simply want to ensure that you uncheck "Move files automatically." Otherwise, it will generate the background tasks to move these files.
There is not an estimator but once you start the process, like with all our background tasks, you can check the progress by going to ACP -> System -> Dashboard -> Background Processes.

To get an understanding of the core database architecture, you can perform a test installation and compare your database to that. I'm afraid, we do not have a listing of current tables.
I say "core", as if you are utilizing any third party add-ons, these could mean that you have more tables than just the core.
While you did not explicitly ask this, we would never recommend deleting actual data, like forum posts for an example, in a case where your database is growing outside of your hosting's limit. Often this comes up from clients and it puts you in a situation where you will be forever chasing your tail and potentially compromising your community. For that reason, we always recommend moving plans or providers (if you can).

You would simply create a new theme by going to ACP -> Customizations -> Themes -> Create New.

This would indicate it’s an issue with your customized theme if it is not behaving like the unmodified theme. Please contact the author of your theme for assistance.

ACP -> Pages -> Databases -> edit your database -> Topics tab -> Rebuild synchronized topic content

I'm afraid, this is not functionality in the default software. While I am unsure of a third party add-on which meets this need, I can certainly move this topic to our Feature Suggestion forum if you would like to suggest this for a future release.

Please be advised I have transferred this to a ticket for you so we can privately handle the backup and it's verification process. Someone will be in touch soon.

Sorry, there are no means to automatically update your community. However, you can follow our Release Notes section to be notified with an email when we do release an update: https://invisioncommunity.com/release-notes/

I'm afraid, our licensing alone, would prevent you from setting up multiple domains on one license. You can only have 1 Live URL per license. Additionally, technically speaking, our software follows that strategy as well as it requires 1 URL.
You could do the redirection thing which @Runar was talking about as that would not interfere with licensing or technical components.

Thank you for reporting this. We are aware of this issue and something which will be solved shortly: 
 

Personally, I would not recommend utilizing a service like that as it is a little concerning linking all external links via it. However, there are numerous areas where you can add in Javascript if you wish, such as the Analytics integration. This will add any Javascript to the header of the community on each page load.
This one is outside our scope of support but you can also modify your globalTemplate in your theme to include this script where you would like. For more on themes, go to: 
 

Unfortunately, this is not something which 2 colleagues of mine and myself can reproduce on your community. I also cannot reproduce this myself on a test instance uploading images as advertisements.
Do you have any one else reporting this? Have you tried this another computer or device? Sounds like this is local to yourself at the moment as we have tried to reproduce this. I can only explain it via a bad connection, bad cache, or something else local to the computer causing this disruption.

Sorry, I am unsure what you mean by this part of your reply. Could you please elaborate?
With that said, I'm afraid, if our software is successfully handing the email off to your email server, as is evident here by the lack of an error, that is where our responsibility ends. Your email provider would need to investigate why the email is being successfully handed but either not delivered or your email inbox is rejecting the email.

With a self-hosted license, you are entitled to 1 Live URL and 1 Test URL (the Test URL cannot be public facing).
You can use your Test URL to test your upgrade process at a different URL than your Live URL. As the Test URL, by policy, can only be reset when the Live URL's domain is reset, I would recommend making this test upgrade instance a sub-domain on your Live URL's domain. E.g. if your Live URL's domain is example.com then I would recommend making your Test URL's sub-domain test.example.com (or similar). 
This way, you do not need to reset anything and once you are ready to go live, you can point your Live URL to where your upgrade is and move forward. Then moving forward, should you need the Test URL, it is available at test.example.com (or whatever sub-domain you chose).

Your sidebar looks to be currently figured to be on top in mobile view due to being on the left-hand side, this would mean all sidebar elements will be above content. If you would like these elements to show up below the content, you would need to swap the sidebar to being on the right-hand side currently.
If you would like to see a change to this behavior, you're welcome to submit a topic in our Feature Suggestions forum.

Unfortunately, I am not seeing any issues. Navigating around your community seems fine. The page loads extremely quickly, the only delay is once the software loads and your advertisements and third party JavaScript loads in.
However, the fact that it is erratic seems to point more towards a server problem than to a code problem. If it was due to a poor performing piece of code, that would be slow each and every time you use that specific page, function, etc... I would continue down the route of your hosting provider as they have the tools and responsibility to troubleshoot this. If this is outlined as an issue with our software, we will require the supporting log entries which they determined this from to advance our troubleshooting.

You would need to ask your hosting provider or server administrator as it is an apache module which needs to be enabled.

The image is set to fit the container with no overflow so if the image has different proportions than the container, it will be cut off.

Glad to see you were able to resolve this and add the correct templates so it shows. However, that is quite odd that "Category 2" was not showing previously. I have tagged this to one of our developers so they can clarify the differences here between the templates you used.

I have converted this to a ticket to have a manager handle the refund for you. Someone will be with you as soon as they can.

By the permissions you have provided the user group, you in fact do want users to be able to read topics but not forums. While I do not believe this is a security issue in the software itself, as the software is doing exactly what you told it to do, I do think that this requires a little review internally as confusion can be had from this if you are expecting something to happen like you did here.

Please be advised that test URLs have limited support. Often if the file set and database are the same, it would be a server configuration item which is preventing it from showing. Looking in your browser console you may see an error. Often this comes up as mod_security when there is a display or save issue which is loaded over AJAX like this is. I would recommend reviewing your server/php configuration to ensure they are the same.