IF it were me - I would junk the "tape measure" retractor and just mount a static cable like every other brand of luggage has. The Yamaha retractor stuff is unnecessarily complex.
Swing arm hitch only works for single wheel, unless there is some magic I am unaware of
3rd wheel hitch is a good example of a single wheel trailer hitch. That is what is on my bike. Fwiw, prices must have gone up. I didn't pay anything near $1000
4 years ago I bought a 2007 for 4K with 16K miles on it. Prices have gone up a bit since then. Understand that 1300 miles on a 16 year old bike is not really a selling point.
Some states used to do dumb stuff like register bikes based on year of sale or mfg date rather than the year that is part of vin. Can make re-registration interesting when the DMV kicks it out because previous title doesn't agree with vin year.
In terms of dry bags, you can buy plenty of no name cheap bags off Amazon that work fine for floating your stuff while snorkeling. The ones we have are Skog Å Kust. If they stay dry in the ocean, likely fine on a bike. Had a Kriega motorcycle specific bag at 5x the cost and not worth the money.
Many years ago a company called pasq started teasing a trailer that they were trying to build. Every year or so, they would post up some more drawings or sometimes a prototype. I thought they had a neat idea, but were taking a really long time to actually build something. So I got bored last...