Um... sorry, but I've never heard of Inversion. Are you perhaps thinking of Reversal?
Reversal
Move type: Fighting
Move power: N/A
Accuracy: 100
PP: 15
Effects: The lower the user's HP, the more powerful the move is.
I'm not sure that Raticate's strong enough to use this effectively. Its speed is pretty good, but I'd worry that its low Defense would get it KOed before you could take advantage of its low HP with Reversal. (And yes, increasing its HP with Rest would take away the power of Reversal.) But on the other hand, not everything's going to have a massive physical attack, so I could be wrong there. Still, tossing in something like Shadow Ball or Body Slam could help you to take advantage of its decent Attack. Even Counter might potentially be helpful.
Of course, I'm not really sure how powerful Reversal can get; all I know is that its power varies based on the HP of the user. I'm guessing it goes up to about 100 or 120 when your Pokemon is at 1 HP, but if the power could be something huge like 200, this could be quite legitimate. Again, I'm not sure exactly what the range is, so my guess could be wrong. But if I'm right (or at least close) about Reversal's max power, I'd think taking a Super Fang into a typical attack/Counter tactic might be a better idea.
As for your questions...
1. If you decide to use Reversal, you might want something to keep it away from status problems, like a Miracleberry. You could also go with a Focus Band to try to maximize the benefits of Reversal. Otherwise, high HP would be desirable, so you could try the standard of Leftovers or use a combo of Rest and Miracleberry (or was there a GSC berry that only healed sleep?) to wake it up immediately.
2. I think Reversal can be a critical hit, as the only special effect of the attack is on its power. However, I'm not sure that Focus Energy would be wise, as you're going to be in a race against time with the SFang/Reversal tactic. After two Super Fangs, I would think your HP would be too low to dawdle with a Focus Energy - after all, what good does a higher critical rate do you if Raticate's unconscious? I just think that that idea might be a tad too slow.
3. I don't really think so. Remember that Reversal will always be strongest when its HP is low - but that's exactly when you'll also be wanting to use Rest. So you'll end up using one, and the other will be a wasted move. Moves that restore HP and moves that rely on low HP don't often mix. It'd probably be best to pick one strategy or the other and stick with that.
EDIT: I forgot this is GSC... aren't I smart!Going back and changing my advice so that it's actually relevant...