Flooring – Can’t sump basin be shut so it can’t overflow

There are typically "sliders" or some sort of adjustment that mark the "start" and "stop" (or "high" and "low") water levels.

Generally there's no minimum low-water level other than you want to make sure that there is always water covering the pump intake (so it doesn't suck air).

The "high" mark just needs to be low enough that water never gets out of the sump. If you have a "high water" alarm (and you should), make sure the "high" mark is low enough that it doesn't trigger by accident.

Other than that, the further apart they are spaced, the less frequently it will turn on and the less wear on the pump (start-up is the most stressful time). Too close together and any backwash down the output pipe could cause it to turn back on again in an endless cycle.

For testing, dumping buckets of water down is fine, as is using a hose to fill it. You just want to make sure that it turns on and off within the limits described above.

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.