Winning with the merchants
I found a thread on BGG where the game designer answers this question.

Do traders work for the 1 to 9 + Inca gold victory condition? In other words, if I have 1 - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 + Inca gold + trader 'X → 7/8/9', do I win the game?
Yes, traders work in that situation. Your trader changes the 1 into a 9.. and you win !
— when do I display the perfect combination? When it is my turn in the progression step (meaning that a combination I simply 'get' from the draw stacks has to survive the trading)? Or can I show it at any one time?
You have to display your combination on your turn during the progression step.

So, yes, you can use Merchants to achieve victory through the full set of resources. Your hand is shown in the progression step, after the trade step.