Yes, but that's separate from what Spectral is reporting. When logged into the AdminCP you cannot change your own primary group. This is largely a protection to prevent kicking yourself out of the AdminCP accidentally and completely locking yourself out.
The behavior you are describing indeed indicates that you've given guests permission to post which you should not do if you wish to leverage post before registering. Instead, you enable the post before registering setting and give the regular members group permission to post - we will see this and initiate the post before registration flow.
If you can't figure out what's wrong with your configuration you're welcome to submit a support ticket for assistance.
We do this with our Community In The Cloud offerings, so I can assure you it's doable. You'll want to store datastore data either in the database or in Redis, and use S3 to store your uploaded files, but otherwise it's pretty straightforward.
Have you overridden your temporary directory in your constants.php file?
define( 'TEMP_DIRECTORY', ... );
Normally the files you are referring to are written to the system temporary directory which would be automatically cleaned by the OS as needed. I expect you've overridden where temporary files are located and that explains why you are seeing these. I can't reproduce any issues in local testing presently.
Actually I think I just managed to reproduce so I'm taking a closer look at what we can do.
No, I'm afraid there's no built in tool to export the data. If you had database access you might be able to use the converters for this purpose, but it doesn't sound like you have that access.
The method you are describing is fine. Plugins can't really add their own classes cleanly like an application can so unless you want to build an application instead of a plugin, just add your methods to an existing class.
Do you get the same issue on a default unmodified English language pack? If so, submit a ticket. But that said, 99% of the time this issue is caused by an out of date translation as Daniel alluded to.
You would probably need to edit theme templates and use the HTML logic you have to add an extra CSS class to the profile photo, and then in your CSS files target that CSS class for the change you want.
I would recommend contacting your host for assistance. A user.ini file isn't necessarily supported on every server - your host would be able to tell you how to make php.ini configuration changes correctly.
I wouldn't worry too much about that - it's not an "issue". Caches are often just duplicating what is in the data store and since Redis can be used for the datastore natively, most things stored in cache don't need to be doubled up.