>QQ for all the pro players out there, for 6 or 9-ring poker, can solvers really solve every scenario - or there is simply too many permutations.
Solvers can only solve using restricted game trees. Generally only heads-up pots, for one--multiway solvers exist but they are cumbersome and there's no guarantee that their algorithms converge to any kind of optimal play. And within the heads-up game tree, you still need to restrict the size of the game tree with a limited number of bet sizes. Not really a big deal, but in theory a perfect poker player would have a continuous range of bet sizes available for them to use (eg 1bb, 1.5bb, 2bb, 2.5bb, etc), not just a few discrete ones.
>The other question I have is how do human players adapt to GTO play now? Like in other games, humans have adapted to novel strategies - I'm curious for poker whether there's ways to exploit GTO solvers (ie, if you put your opponent bot on the GTO range and chase the fat tails).
You can't really exploit GTO solvers. But a human player can't play a GTO strategy, it's simply impossible. People can get reasonably good at playing a few of the most common nodes in a low-exploitability way, but beyond that, nobody is placing a perfectly balanced strategy. It's just impossible. The strategies are way too complicated.
>And is the poker community's fair play detection algorithm good enough to catch players who use solvers like chess community - or it's easy to evade by making a few deviations.
The strongest fair play detector right now, at least at high stakes, is the players themselves. Precisely because playing a GTO strategy is impossible, if someone starts playing too perfectly, other regs will notice. And the high stakes community is pretty small too so everyone knows everyone else, or at least knows who knows them or whatever. Those two things combined mean that there's a lot of self-policing.
Also, playing a GTO strategy isn't necessarily always desirable. Equilibrium strategies are fundamentally defensive. A solver will defend at the perfect frequency against a river bet: folding just enough to make you indifferent when bluffing, so that "always bluffing" and "never bluffing" have the same EV (this is technically oversimplified but whatever). But this approach doesn't make the maximum amount of money. To win the maximum, you want to just always fold if your opponent isn't bluffing enough and always call if they're bluffing too much. Of course this opens the door for you to be counter-exploited (if you always fold, they can start bluffing all the time, and vice versa) which is why the solver lands on the goldilocks equilibrium.
In other words, a GTO strategy does not punish frequency mistakes, and Hold 'em is a game of frequencies. Of course a GTO strategy will still win money, because people don't just make frequency mistakes, they also make just straight EV mistakes. So there is certainly some risk of being cheated, sure.
Another thing that disincentivizes cheating a bit is that you have to be extremely consistent with it. In chess, using an engine in just one or two critical spots could be enough to swing a game completely. In poker, because of the imperfect information & randomness, the wins and losses are distributed far more evenly across time. It's not enough to just check a solver a couple times a session if you're bleeding money in the other hands.
Anyway, all of these are real issues, but the bigger issues facing online poker are all just related to game availability and quality: poker sites charging way too much rake, fewer recreational players willing to lose a lot of money online over time, a gazillion legal/regulatory issues that make it hard for people (both professionals and recreational players) to even play online in many places in the world.