As it stands, I can see 4AP being equivalent to two Half Actions. From there, we can extrapolate talents and bonuses and whatnot. Am I missing something or is it this easy?

Sadly I don't think it's quite as straight forward as that. Because you need to have AP for reactions, whereas in DH1 you get a reaction for free, it's more like:-

4AP = 2 Half Actions + 1 Reaction.

1 Reaction being 1AP means a HD1 Half Action = 1.5Ap in the new system.

Talking very roughly of course.

Rats.

Not to mention weapons, like a great weapon, that take 3Ap to use....

It would great if we can find a way to do this ASAP though.