It is good practice to get an air change in there every few weeks or so. I have also poked small holes in one or both sides of the bag to allow a bit of air exchange, but I don't usually do that. Either way, watch out for mold and the like. I have never seen any--probably because of the antiseptic properties of sphagnum--but I understand it can happen. A squirt of neem oil or sulphur-based anti-fungal in the bag is good practice if you have some lying around.
They are hard to find in the moss come spring. In the past, I just "milled" the sphag in the bag by hand. I removed all the air, then just rubbed it around a lot until it was all fairly uniform. I then spread the seed/moss mix on the top of an eight-inch pot of peat and perlite. Since you are trying to place them in individual trays, it is a bit more tricky. I have little experience with that. I actually just sowed a lot of leucophylla seeds in seed starter trays last month and have them stratifying in the garage. This is my first go using the one-per-seed-pot method. I usually have just gone communal and planted them out separately later. They stay tiny for so long, you know?