does salt go bad

If you were keeping your tank at 1.026, then I would worry a wee bit. If you were keeping it at 1.022 (which is fine for Fish Only systems) Then I would slowly bring up the sg over the course of a few weeks.

As long as your nitrates aren't sitting at some ridiculous level (like 120 or something crazy) I really wouldn't worry about the cuc. Just acclimate them for a bit instead of just dumping them in.

I've been fighting high nitrates and haven't had any cuc deaths caused by it. Caused by a hungry brittle star... but not high nitrates. Mine were off the api test chart when i first got the tank, and now are around 40-60ppm.