Is it fair to use gear no longer available, since it can give an advantage over new people that will never have the gear to compete against times set by gear that is currently not available permanently (if that makes sense).

There aren't really any up to date guides, though something I feel would be important to tell you is to make backups of a run-ready save before you try some of the strats (you can softlock), and that 1.00 and 1.02 are best for Any%, AGB, and 100%. 1.00 and 1.04 are the best versions for NG+ Any%. Resetting is also more annoying than in Nexus, I'm afraid.

There's this outdated glitch spreadsheet: https://docs.google.com/spreadsheets/d/1bf0JM1OR_PrXEaYhD2MmduFtivH4F7MA9eAnxpgeDZ0/edit?pref=2&pli=1#gid=0

Yes. Though doing the old tryouts skip was made impossible for NG categories in 1.04, if you were trying to damage boost from the lamp.

So you can compete for fastest Korgon completion (and other levels).