Just watch the video :) Hollow body position is absolute must for flyers and must for bases.

TODO: add hollow body drills.

You don't need a strict press handstand to start working on h2h because base can help you to get all the way up. However you need a good form which enables you to go straight up without having forward momentum (which then needs to be stopped which makes it hard to keep the balance).

Check out Erik's handstand drills channel Hold the earth (demonstrations by amazing Nataly).

It's enough to be able to do assisted (spot, safety lines, soft mat etc.) front flip, the main purpose is spatial awareness while flipping because there's lot of front flip like movements in acrobatics.

Same as with front flip, you need good technique but assisted backflip is enough. Focus on spatial awareness in the air.