Bumpkin, heeling the boat over with a line form your masthead to another slip or such is a perfectly traditional way to get a fitting above the waterline.
Beeswax makes a fairly good temporary plug, and it is easy to "chew" and wipe back out of the fitting afterwards. But I was thinking more of using it as a backup for your epoxy repair, than as a primary plug while you were spinning fittings off.
The plugs are usually available in a poly-bagged four pack from all the boat supply stores, of course at least two of them will fit nothing on your boat "one size fits all" haha. They're usually "NAFTA" packaged for the US/Canadian market, so the French side calls them "tampons", apparently that's simply French for "plug".
At first i couldn't figure out the pcitures you posted, then I realize the u-bolt and grey glob are the PATCH, not normal equipment. Ideally the shut-off valve should be as close to the hull as possible, so there's less chance of someone standing on it, or something banging past it, and shearing it off. I'm not sure what all that plumbing there is doing--but anything you can also so to reduce it and make it less vulnerable, counts.
Of course whatever parts you need, from a reputable source, in the right material, will only be stocked in the wrong thread size or special ordered from a craftsman in the far north of Finland. that's just the way marine plumbing goes--be prepared to make multiple trips, multiple measurements, and wait a week, twice.
All good reason to "do it right the first time" !