Nope. You can't sway opinions, you can't foment revolts and you can't shift power groups with Wealth. Ordinarily you can't even pull an alliance switch like that, it evolved naturally out of the treachery.
And, since it hasn't affected their opinions, those new allies are about as reliable to Primeria as they were to you. Influence can improve relations to the point where betrayal isn't a concern - at Loyal and Fanatic relations, NPC allies will not betray except under special circumstances (e.g. if you are clearly and obviously screwed and their best chance of survival is treachery - and even then Fanatics might stick with).
That said, trading resources is certainly powerful simply because trade doesn't have a PC cost. Let's say you have 100% cost for raising Might. You need to spend 2 PC and 1 Wealth to raise 1 unit of Might and then send it off to war. Instead you offer your AI ally Generica 2 Wealth to do it instead. They take your 2 Wealth, spend 1 Wealth on raising the Might and then send it off to war in your stead (costing them 2 PC, but earning them 1 Wealth). They might charge more or less depending on their attitude toward you.
The middle road is a faction like Mecklen. Mecklen spends 2 PC to recruit 2 Might (at a cost of 2 Wealth), gives them both to you for 5 Wealth and then you spend 2 PC of your own to send them into battle. Mecklen loses direct control of its mercenaries when it hands them over, but doesn't need to spend the PC to do the invasion either. Mecklen is basically just an arms dealer in the transaction rather than an independent entity - which is why Mecklen's mercenaries couldn't betray in that last exchange (though they were certainly approached!).
Victory. We had won. The Albians gave us a hard fight but somehow we survived. Treachery is something that runs deeply in these lands, men turn their cloacks ant every opportunity for coin. And thanks to our Allies we survived. The Jarl and all of Finster would not forget the sacrifices our allies brought. Thanks would be forthcoming soon enough when they would be showered in the wealth of our raids.
Question: Now that it costs no PC to let troops raze stuff, do i still use up my Might units for razing or do i keep them?
Use 1 Influence to increase support of my Powerbase
To clarify, you still use up the Might units razing/capturing, yes. It just costs PC to move them into the theatre, but since you need to be in the theatre to do the razing...