Was there any way Elan could have talked Tarquin out of this?

[veti]

Malack didn't actually convince Tarquin of anything there, remember? Tarquin went right on planning to arrange matters so that Nale and Malack had to go back to working together.

Well he convinced him to apologise to Malack. That's something, even if it was about as sincere as a succubus's kiss.

I think it's entirely possible Elan could have said something that would have produced a different outcome. Something like "Dad, you don't get involved in every combat, that's what fighters are for! A hero only gets his hands dirty at the appropriate climactic moment."

But since he had no idea what lunacy was going on in Tarquin's mind, he had no way of working out what he needed to say to prevent it.

Personally, if I had to guess what will save the party this time, I'd put my money on Laurin and/or Miron to speak up. They don't share Tarquin's narcissism - at least, not as it relates to him - so it follows they don't share his view of Elan. More likely, they would see the OOTS in the in-world rational way of seeing them: as the best, possibly only, thing standing between them and a scenery-chewing, world-dominating threat.