The angels should be as good as their hosts are. After all, they share memories and feelings, so each of them should behave in a somewhat similar way to her goddess.
It's indeed true that they can change hosts, but the process can put a lot of strain on Keiichi's body. Still, something of that nature might actually happen, although not because the goddesses gave their angels to Keiichi.
Now, for Hild's avatar… The truth is that I'm not comfortable with minor characters (even if it's only their appearance and not their actual age) in a story like this. However you might indeed see Hild (in her full body) in that outfit, and her habit of kissing others out of the blue will remain.
As far as that outfit is concerned, there are two versions actually. One is what she actually wore in that race and the other is part of a cover from the manga. Here’s the images:
In story:
And he’s the other one: