Lightroom Classic Catalog unneeded files

Yes, all your edits are carried over to the new catalog. 

Yes you can (eventually) delete all the older ones. Bug I agree with dj that you should back them up for awhile until you are certain the new version and catalog is all fine. V12 is the latest and those from older versions (V10 as an example) would need to be reconverted to V12 to work, or you'd have to revert to an older version to use them. So at some point, you can trash them. 

You can look at the contents of this new catalog and see for yourself. Just double-click on it.

Honestly, I never advise people that they can delete catalogs without problems. Some people here do advise that. But I don't know what is in your catalogs, even though I *think* I know. You would be wise to look at the catalogs yourself and see what is in them, before you decide to get rid of them; or vice versa ... if the newest catalog has everything then you don't need the earlier ones, but YOU (not me) have to determine that.

Alternatively, you could move the catalogs to an external drive and save them just-in-case (you can delete the associated preview folders).