Re: Mercenaries
The more I think about it, the less likely he's managed to get significant, much less reliable hirelings from Water. And if he hired True Dragons--and I really doubt that, since while we make interacting with random True Dragons seem easy, we ourselves are not only a Huge-sized chromatic, we are a Mythic Dragon Archmage on top of that.
Respect is a huge part of Dragon Diplomacy, and I don't see Golden Shield envoys being able to make it through a single sentence without stuttering and startling while conversing with one unless they were Companion-grade. Much less, again, be able to offer a high enough bounty on our head.
If one was impulsive enough to take their first offer, it was probably some young Brine. We could probably arrange a meeting with them and quietly pay them off and call it an amicable business meeting over brunch.
Secondly, we are offering literally any mercenaries in Water, from independent mages, entire free companies, what have you, an offer to fight. With contracts that just can't be beat. With the implicit implication of our direct support and involvement in any fighting.
Tywin has to either cut a deal with the Brazen Throne or Baator. I can't imagine anyone who would place their involvement as beyond the means or reason of profit, but rather a position of utter confidence and power (Baator) or supreme spite (the Sultan).
We can be reasonably certain he has been putting his feelers out toward one of the two without Lanna or Gerion being aware, if he's seriously considering outsourcing his Targaryen problems.