How do I learn to effectively transfer to 2v2 hard AI as Zerg?

First,

initial rush survival

Spinecrawlers and properly microed speedlings and roaches can really help in pushing back this initial rush. Force the AI to funnel up the ramp and place the spinecrawlers behind gas and/or an evo chamber. If you scout it early, set your speedlings outside your base so when they attack you get an easy flank. Use multiple queens with transfusion to keep everything going. Remember you can use them even on your spinecrawlers.

In regards to

What t3 units complement a hydra-heavy build?

I think there is a fair amount of discussion about this but I personally find a lot of use with broodlords. A tier 3 unit, which is very powerful at wiping a ground army. I prefer to use them in team games as well. Excellent base assualt as well as support.

EDIT: In a recent match between Idra and Drewbie(?), Idra tech switched from Ultra to Broodlord and ended up finishing the matched. Had he switched earlier, it is speculated that he would have much sooner. More power to the broodies!

1) Personally, when I'm playing Zerg, I put a couple of spine crawlers up as defense, and that's usually enough, but I'm also putting a lot of pressure on their base with my attacks. I've found with zerg, I'm much better if I play more aggressively than Terran or Protoss.

2) Corrupters work well with Mutas. They still have some value without any air enemies because of their corruption ability, but then I ramp production of them up as I'm prepping for brood lords.