6x 0
1x +1
1x +2
1x +3
1x +4
That would get you a Ghostblade +10, all the pluses of the ingredients are added during fusion. To get a +15 in that case with a Water Emblem +1 you'd need a Cultist's Dagger +4 which would suck to grind. I recommend you get some better Air Crystals.

A Ghostblade +14 and a Water Emblem +1 would result in a Master's Dagger +15, yes.

All customers are only limited by their budget if they either ask for a type of item without picking it out, or through an advance order. If they take it off the showcase and go "sell me this", they will always be able to afford it. If you want to force an adventurer to buy something, then put it as the only thing in the store, adjust the atmosphere to their liking, save before opening the store and reload until they buy it or you at least have an order for that type of item.