If martin originally wanted to end a dance with dragons with the battle of ice concluded, wouldnt it make more sense from a narrative point to have stannis lose? If he won, then we know exactly which part of the pink letter were false, while if he lost, then there could still be much uncertainty about the letter

That assumes that the narrative point of the pink letter is to create uncertainty. I would disagree with that – the narrative point of the pink letter is to be the catalyst for Jon’s death, the uncertainty is all meta-narrative. 

Moreover, the importance of said uncertainty is highly contingent on the letter coming at the end of the book – thus setting up a cliffhanger mystery – which wouldn’t be the case if the book ended with the Battles of Ice and Fire. Instead, the letter would function at a meta-narrative level as setup for the payoff of the battle.