First, that's a pretty big assumption.
From a Doylist perspective, there's not much point in bringing the Eebs back unless you're establishing that the Ramps are still going to be around. Therefore, them surviving and them being able to reproduce are part of the whole.
As for them being more powerful... eh, I'm not totally sold on that. They have a few crippling and -- most importantly -- well-known weaknesses that let pre-infection Susan take a few of them out.
It's said in the books that the Wamps are inferior to the other vampires, strength and speed-wise. As for Susan, I think that's a stretch. Pre-bite, she used holy water and a gun to likely kill one. She helped the others keep some at bay. That's all that said about that.
Considering that they were obviously trying to take Susan and Justine alive (because they did) it changes the dynamic of the fight. They weren't going for the kill.
Truth is we don't know what all their servitors can do. Having minions that don't have glaring weaknesses or an addiction to blood might be worth the trade-off of power.
I'm willing to bet there's some that are more formidable. But as general shock troops, do you think the ones we saw so far would be better than Ramps? Short of taking out their blood bladder, Ramps are hard to take out. The servitors so far fell like chaff.
If there were more than the two most cowardly examples of the species, I'd agree with you more.
I think your definition of cowardly is odd. Staying alive to complete your mission is an interesting take on cowardice.
And if Harry hadn't had the get-out-of-paralysis-free card to play, they would have
won. He had to sell his proverbial soul to overcome their machinations.
Think of it this way, though. Let's say they show up in Peace Talks on official business. Harry knows where they were last seen. He's on speaking terms with the guy who had them. You don't think he's going to ask? You don't think everyone else -- like the White Council contingent -- is going to notice that the Fomor had been allied with a group that had so recently tried to genocide them?
The Fomor or Nemesis would have to show a few of their holding cards to get them out and keep them out. I don't think it's worth it.
I already provided two examples of how that could go down. One is that the Erlking lets them go to preserve a hunter species. The other is that the traitor has them freed and frames someone else that won't be around to answer questions. Some minor Lord of the Court is convinced to do it, and is then betrayed by the spy and knocked off.
Erlking is left with suspicions, but nothing to work with. And if he wasn't already keeping a suspicious eye on his people, given all the infiltration lately among the Sidhe Courts, he's an idiot.
What if they're planning to use the peace talks to demand restitution for, you know, genocide?
Mab doesn't seem like the type to write in a provision letting the losers get payback for losing a war they started, with a weapon they created themselves.
Mab, to me, seems much more likely to include language in the Accords to the effect of, "You brought it on yourselves."
I concur that restitution is unlikely. They could arrive as part of the Fomor party, or they could arrive as survivors that are looking to get some minor Accords protection as they re-establish their Court, conceding that they're done with war and revenge. Until they're not.