I want to say I read that doing disc2app allows you to play installed backups of your physical games without needing
sigpatches for that, even being able to play without custom firmware running -- is that correct?
Should be correct, yes. But WUDD is the new disc dumper with active maintainance, so better use that.
Are these real disc tickets unique to each game, or to each disc even?
To the game (they contain the title ID. If you change the title ID the signature doesn't match anymore, so you need sigpatches again).
if you deleted the NNID, could you use NUSspli to create fake tickets for those installed games and play them right away, or would you need to use a ticket cleaner first, or even delete them and redownload them through NUSspli?
Good question. The Wii U saves all tickets, even for the same title, and uses the first one found (which normally means the first one which was ever installed on the console). That's why the free DLC trick works in the first place.
But this also means if you reinstall a game it will still use the old ticket (except when you do it with NUSspli: NUSspli does delete the ticket while uninstalling).
NUSspli can create fake tickets only but it won't exchange them in the ticket bin of your Wii U. For this you have to reinstall the title (again: With NUSspli. A normal uninstall won't delete the ticket but NUSsplis uninstaller will... Or exchange the ticket by hand but that has a brick risk, think about accidentially corrupting the ticket of the Wii U menu or something just because both tickets where at the same file. Yes, the Wii U combines multiple tickets into single files).
//EDIT: As a side note: When you try to install a title with NUSspli but that title is installed already it will uninstall before reinstalling. Originally this had been added to make Channel updates a no brainer (WUP Installer corrupts them when you don't uninstall manually before) but now I see it helps with ticket exchanging, too.
//EDIT²: Yes, NUSspli has a lot of hidden features most people won't notice until they need them.
//EDIT³: I said the ticket contains the title ID before but forgot that it also contains the decryption key for the .app files. This key itself is derived from the title ID, so you would have to change that one, too, or the Wii U won't be able to decrypt and as a result see the .app files as corrupted.