A little goofy, some things to fix

I'll take your opinion under advisement lol

I'm not dead set in shortening it. Realistically, the likelihood of me reducing the length by any significant amount is very low just because of how much work it'll be, and I've got like three days of interest left in this project before it gets abandoned like so many others

Not to be that guy, but fixturing/jigs is one of those spots where a 3d printer and cad skills could come in handy. It would be pretty straightforward to draw up some blocks that support the fins and you could put a band clamp around.

How about - use an offcut that the body sits in at an angle such that the fin-body join is horizontal - the fin being in the clamp. Apply pressure with a weight on top and use masking tape to hold loosely together?

You can also use fine saw dust preferably from a hardwood. Works about as well.

Salt will dissolve after some time, so it's imperative to get the rubbers on quickly. You would use salt over sand if you planned on sawing it apart at some point, since sand would dull the blade.

it's a SUPER old school trick to keep things from sliding when gluing up. The intent is that the salt is there long enough to form some grip, but then dissolves in the glue as things set up. The more modern way to do it is to put a dab of superglue on part of it, and wood glue on the rest and the superglue sets quick while the rest dries more strongly over time. You can even use the superglue activator to speed up the process. Not sure it'll work great here cause there probably isn't room to do both in that small area, especially if the compound angles don't all match up 100%.

If it were me, I'd put some glue on it, then shoot a small pin nail into it to hold things together long enough for the glue-up. A pin nailer is invaluable for assembly.

If you have an air compressor, grab one of these https://www.amazon.com/EASEWORK-Pneumatic-Headless-Creates-Invisible/dp/B0GSTY13TG

If you want to do it all by hand, a pin vice (handheld tiny drill) can be used to create a small hole and then you can push in something small, like a toothpick to hold it in place long enough for the glue to set. They're designed to be used one handed while you hold the thing in the other hand. https://www.amazon.com/Vise-Hand-Drill-Jewelry-Making/dp/B098CF3VMM

*two axes

A plane can only have two axes, i know things

Drill the dowel holes in the legs first, place a pointed plug in the holes (several companies make these you can also make them your self with some rod and a grinder) then mark up your rocket to align your legs and press them into it leaving perfect location marks.

There are then several different ways to go about drilling a square hole into a cylinder.