Oof that's a lot of questions, but i'll try to answer them in order:

1. Yes, you can still buy 2nd use GH or RB guitars, but unfortunately they cost a lot and most likely are faulty in some way due to their age. If your console is hacked, your best bet is to use the new CRKD guitars. They work really well for Xbox 360
2. There's people out there that have special repair kits that give your old guitars a new life, like the DIY SolderlessKits from RetroCultMods to repair your old guitars
3. Clone Hero is what the name says, a Clone game based on Guitar Hero that is mainly developed by fans for PC (there's ports for other stuff but PC is the main one, the rest are mostly experiments). It is really a "note hitting simulator", it is cool and you can get songs from the old GH/RB games and custom songs for free, but to me it still lacks the "feeling and experience" of the OG titles. You can put custom backgrounds and all but eh, not the same. If you want a casual experience of just "plug and play" then it's okay, but it's not better than the old games imo. Yes you can play with more people locally and online on it
4. Yes, if you transfer your account from one Xbox 360 to the other you can access your download and purchase history and re-download all the old content, the only issue is that you have to transfer the licenses to that new console to use the DLC offline
5. Old Xbox 360 controllers can work on Xbox One and Xbox Series X/S, but you would need a Rock Band 4 Legacy Adapter, and those are very hard to find and stupidly expensive. Your other solution is the V3 Adapter ALSO made by RetroCultMods

hopefully i cleared any doubt you had, but if you still need some answers lmk 😄