Merling should be able to use cauldron or water bottles to refill air in nether as there is no other way that seems viable.

All the ways a non-Merling can breathe longer underwater, Merlings can use to extend their breath on land. Use a respiration enchantment and a turtle helmet, alongside drinking water bottles, to make your first trips to the nether. Once you have water breathing potions, you're basically all set. :)

then maybe buff air gained by bottles? I tested it and 2 is bit to little how about 4? when I actually gain the breathing potions it'll be useless as I won't need them anymore when I gain blaze rods