How to seal sump pit with removable cover

As kkeilman points out, your pump is fully submersible, so that part should be no problem.

You can easily just piggy-back a float switch like this one on your existing pump. It has a special plug that you plug the existing sump pump into (no wiring necessary), and this one in particular handles up to 1/2 HP, which is more than enough for your 0.3 HP Zoeller.

The nice thing about these tether floats is you can adjust the heights depending on where you attach it, and how much tether you give it. Just be sure to test it manually moving up and down to be sure it's at the right heights, and it's not going to get hung up on anything (causing it to either stay on, or not turn on when needed). I highly recommend you secure all the wires going into the pit with zip ties or electrical tape to be sure they're not going to fall down in the future.

You can leave the existing float working, which is fine - it will act as an extra safety to shut it off if the pit is dry, but also be sure this one doesn't get hung up - because both floats will have to be "on" to make the pump turn on -- if either is off, the pump is off.

If you don't want to use the existing float, there may be a way to bypass it internally by rewiring the pump, or you can simply zip tie it "up" so it is in the on position all the time.

Current code leans to GFCI the heck out of it, and if it nusiance trips or fails (as they do with some regularity; typically about 10 years) and backs up sewage or water, tough noogies. People who sell GFCIs write the codes, and they have been expanding places where GFCIs are required for years. As of 2002 code (at least) there was still an exception for sump pumps and freezers, but those have been removed due to either "greater care for your safety" or "vast sums of money from GFCI makers .vs. no sums of money from people with flooded basements and freezers full of rotting food" in the code-writing cycles. [It should be self-evident that the linked page is from the side with all the profit in this game, and just a tad smug/biased.]

I would NOT change the ejector pump to a dual, nor add anything to its circuit - sewage backups stink. A dedicated breaker with one outlet is one way to be certain that nothing OTHER than a pump failure or power outage causes the ejector pump to stop; and depending on the pump, it may need the entire circuit to itself, period. Even if it does not, sharing is poor practice, IMHO. If you would like to add a GFCI to it, put in a blank face GFCI in-line before the outlet, or a GFCI breaker. If it's installed to code as of the time it was installed, I'd suggest leaving it alone, but that's up to you.