Q's 2 and 3: Try it. you seem to have your setup for slot cutting. Just al long as you can get long slots to stay straight. Might try making foot long slots with a gap of 2 inches between them. This would keep the long tube from opening up wide at the slot, and collapsing when flexed.

: 4. There seems to be too much wiggle side to side right now. I'm not sure if
: this will be solved by completing the keel/stem connections or if I need
: to be concerned about the boat's proclivity to shimmy side to side.

I think this will be reduced when you put in floorboards. You can get a lot of crossbracing from a big sheet of floorboard, while little pieces will let the boat flex more.

PGJ