Oh, 'Rocket tag' is the idea that the first person to have an attack round wins, so the focus is on winning initiative. A rocket proof or rocket resistant build, rather than beat initiative, focuses on surviving attacks, or at least the first few.
Let's go through it step by step, and you can find out.
Not according to my PHB. You can't get a bigger effect by burning xp, you can get a bigger effect by actively inviting the DM to screw you over. Not something a paranoid wizard would attempt. Not twice anyway.
Try it and we'll see. I'm not invulnerable, noone is, but I think I've got a decent chance.
Of course it's not fair. We've set up a situation where the tier 1 caster is all alone and being attacked by a high level character. Why would it be fair?
Pay for the sevices of a high level bard? It won't get the spells you've learned today, but it would get the effects you've come up with in the past, so some idea of what is in the spell book, some major items, tactics you've used in the past, etc.
If you're 20th level, you have a history.
I'm happy to trust you on the build if you trust me on mine. Don't forget to pay list price for every spell in the book though. We both have ways of exploiting wealth per level to get free spells or cheap stuff, but that's a whole different contest.
If you can hit, sure.
Well, there's having a high touch AC, there's being hard to see/find, and there's immunity to mind effects. Just as an example, an undead rogue would just laugh off such an attack.
I'll sort out a build over the next few days, and we can try describing what we think would happen. Maybe you'll surprise me.
It may end up being difficult to model aspects, but I'm curious as to what you come up with.