Here's how I see the manipulation thing:
Assuming that hiver "manipulation" is basically highly advanced and very effective Memetics (I'd rather it was that rather than some magical alien superpower), the Solomani are going to be pretty good at it too - hell, we're fairly good at it today.
The Hiver vs K'Kree incident is basically an extension of the arguments a human meat eater would use to convince a human vegetarian to eat meat. Except they're much more persuasive, to the point of convincing a herbivorous
species to eat meat. I don't know how the heck they did it, but I suspect it was a fairly long-term, focussed effort (do I recall correctly that it involved Hivers brainwashing K'Kree prisoners and dropping them back on K'Kree worlds to spread the word?).
Solomani of course would be wise to such things, since they'll undoubtedly have practised similar techniques in the past. Today, everything from advertising to selling slimming and cosmetics to swaying people to vote for or against a political candidate (or for or against blowing themselves up, for that matter) is down to some kind of memetic manipulation. If we're doing that today, then the Solomani must sure as hell doing it in the TU (especially given their "The Party Is Great" mentality).
Hence why it's going to be an interesting environment. Assuming that both sides are interested in swaying the people between then, the people there are going to be bombarded from all sides by very effective, focussed advertising and propaganda. It'd be like the Cold War, but more intense (yet still not involving actual firing of weaponry). Which makes me wonder - is that a stable situation? How long could this go on for?
Of course, that assumes that either side wants to make the effort (if one does, the other will follow). It may be that some worlds or polities between the two may even actively shut out both sides, effectively sticking their fingers in their ears and saying "lalala, I can't hear you!" to stay out of the fight