Re: transom replacement 21 ft alum boat
I'll comment on what I know....
#1 I would do a single U and fit it in. That should be strongest transom. I hadn't actually seen an SC with a pieced transom before yours.
#2 The best way to do a deck if you want it last a long time is to seal the bottom and edges with epoxy. Then install it, seal the top with epoxy and if you really want it to last, add a layer of class. You can then finish it with whatever you want. Your no-slip suggestion would be great. I'm doing the same thing right now and will be doing all of that plus using a no-slip product called Durabak (or something equivalent) on the top.
#3 Ah, bummer. You have one of the newer SC's with the poured in foam. That stuff is a mess. Chances are good that it's waterlogged and really should be removed. That's not a fun job. Many of us (including myself) are replacing the soaked foam with cut up and pieced in pink or blue construction sheet foam from Lowes/HD. It will NOT absorb water.
#4 Only suggestion I have would be a skim coat of JB weld. I had the white stuff on the inside of the transom on my jupiter. I think it's just calcium deposits or something.