Richard,
And you're back to tradeoffs. 9, 10, or even 15 mph stall speed is not a death knell, but it might not suit what you want. Your current build can fly, but you're right -- trimming weight can give you slower stable flight, which would better fit your final plans. Higher weight (wing loading), however also lends to better immunity to the wind, so each choice may hurt in one way and help in others.
Either 700 or 750 motors should be ok, but you're probably going to have to move them farther out on the wing to hang the props. For a given pitch, Props will only generate so much thrust for a given length and RPM -- too short of a prop on a big motor, and you'll never get a good efficient power transfer to the air. Check the specs on HK or online to see what props are suggested (might even find thrust measurements by prop size from users).
As for lighter materials, haven't found good ones at the local hardware stores. Aluminum is good, but if you really want to cut weight and keep strength, try hunting, golf, or fishing stores, looking for CF or fiberglass shafts -- or find them online. if you overlap and bind 2-3' shafts with Kevlar or spectra thread and soak in thin CA, you can get some wickedly strong/long/light shafts. CF has higher strength per weight, fiberglass is heavier but much more resilient and flexible. Also, the strength you need can be from tiny diameters from these rods, so don't look so doubtful at that 3/8" rod (or even smaller).
Check around for prices -- it can be super expensive and super cheap. I've found it cheaper sold as an unpurposed material than as a "golf club shaft", but I've seen sales from time to time. Only online place I can recommend offhand is R2 Hobbies out of Taiwan. Shipping is expensive, but the CF is in 3ft lengths and more than cheap enough to make the difference if you buy any fair sized quantity. Plenty of other places sell it and if you find somewhere local, may be worth a few dollars more.
Switching to a single boom has advantages, lighter weight being one, but you loose a lot of twisting strength between the wing and tail. Hard to know how much strength you need until your tail is flopping in the breeze, but if you don't need it, you don't want to overbuild . . .
One last comment (feel free to ignore). You seem to being enjoying the build (a good sign -- if you fly, you'll have to fix), but you're working hard to get that perfect long term plane. By all means, keep working on this -- but perhaps you might want to put an engine, radio, and a small battery on that noob tube and get it flying. I'm enjoying bouncing ideas and seeing the directions you take in the build, but flying and swapping out props/motors/batteries really does help get a feel for the tradeoffs -- what do you get, what does it cost.
Time for me to practice what I preach. Got a build to clean up and get in the air!
