Monterey and Photos

Yes it will all stay the same. Very rarely things can go wrong with an update - which is why a backup is important.

I have not yet upgraded to Monterey. I don't feel I am missing out yet. You don't need to upgrade if there is nothing in Monterey that you need.

Photos in Monterey is basically the same app with some feature enhancements. For details, see the photos section here:

macOS Monterey - New Features - Apple (UK)

If you have a full backup, you can re-install an earlier version of MacOS, and then restore all your files from time machine.

However, if you are going this route, you should take a second backup, as while you are re-installing MacOS, your system drive is erased, and then your time machine backup is your only copy of all your files - if that were to fail you would lose everything. You could use (for example) carbon copy cloner to back up your complete system drive to an additional hard drive.

See also

How to get old versions of macOS – Apple Support (UK)

Yes you're wrong in assuming this.

But, as always, make a full back up before updating. That way if something goes wrong you're protected. And, if you don't like something in the new OS, you can revert.

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