Had Daeron I managed to escape being killed in that ambush, how likely would it have been that he could have won against the Dornish? He has the incident of abusing the peace banner for rallying the nation, and seems to have a good mind for military matters (if nothing else), but it does seem like his odds were limited from the start with that guerilla campaign. Was there a chance for a victory, or at least not a defeat? Or was the campaign doomed from the start?

Ultimately, I think it would come down to how much punishment Dorne was willing to take, and what outcomes each side was willing to settle for. 

On the first hand, I think we under-estimate how much damage was endured, even by successful guerrilla campaigns – remember the whole thing about the 50,000 spears of Dorne being actually 25,000? I think the Dornish lost a lot of manpower, sufficient to actually cause long-term gender imbalances and lower birth rates. After all, given Daeron’s military prowess, I doubt the 50,000 Westerosi lost in Dorne were the more numerous, and Dorne suffered civilian losses on top of that. So…how much punishment were the Dornish willing to take? A lot, clearly, but not an infinite amount, given their willingness to seek peace later. 

Regarding the peace outcomes, I wonder what would have happened had Daeron I offered something like Daeron II’s deal. And Dareon I was still unmarried…