you do need to attach the L on both sides, so why not just use screws for both, vs having the top of the bolt on the surface of your shelf?

so, where are you attaching the bolts and nuts? and how? got a drill?

Brackets will work, with screws. But if you buy a piece of lumber, the store usually will cut it for you if you need. Also, if you have a house, you need basic tools. Scope out yard sales, auctions, estate sales or thrift stores if the budget is tight. My fave is auctions: I've gotten boxfuls of power tools and workbench stuff, sometimes for just a couple of bucks.

For the floor, you need to first figure out where the water is coming from. Please tell me this isn't built right on the ground with no structure underneath. Regardless, rotting wood isn't going to heal itself so you might need to start over and do it right.

For the gaps, if you don't care how it looks, a can of spray foam (like Great Stuff) can fill the openings. Or use painter's caulk.

For stabilizing the wood shelves, cut a small block of 2x2 wood the width of the shelf and attach it with screws underneath at the corners.