I believe that 1/4" carbon tubing x 1/32" wall has plenty of strength as longeron stock and is very light and stiff, but bends enough to conform. I think the longest I've seen is 4 feet tho. The lonerons will need splicing, A 1/4" ID carbon tube that the pieces are inserted into will work fine. Six inch splices are long enough.
At the duct areas of the fuse, I'd suggest doubling up the 1/4" longerons.
On wing loading, yes true enough, IF the model weight comes in at 30 pounds. i was guessing that you were going to shave 5 lbs off the total dry weight. Of course with jets I forget that take off weight is about 1/3 higher than dry weight. Talking shaving weight, have you considered the tank types yet? Let me suggest clear PET plastic types rather than the epoxy-kevlar-glass. Fittings can be obtained at Eldon James (internet search). I sell the 1/8" size but you may need larger ones that I don't carry.
PET is polyester and is impervious to kero, avgas and auto gas. Many drinks come in 64 oz bottles so finding that size should be straight forward. These tanks are bound to save you as much as a pound of dry weight.
Thust versus speed versus load....that's a rather large wing. I hear what you're saying about the two 2" aluminum tubes. I was thinking one 2" carbon tube with a thicker wall, around 2mm or so. If a 7/8 x 1/32" x 36" carbon tube can hold two full grown men (at least 350 pounds) then a 2" x 2mm x 40" carbon tube should hold at least 600 lbs or 20+ G's of loading. The weight savings would be significant too....Of course 2 7075 ally tubes will cost maybe 40 dollars where the carbon probably at least 3X that.....Just trying to inform you of other possibilities....cost would go up of course but might be manageable
Re: wings, ailerons, TE's etc. The foam wings i prefer are cut with the TE down to the desired thickness, usually no more than 1/16". Sheeting adds another 1/8". I usually cap off the TE with med-firm balsa and feather the balsa accordingly. The foam TE usually will have some 3K carbon rovings, top and bottom, laid flat with epoxy. I cut the ailerons out of the wing. Aileron and wing facings are usually 1/4" medium balsa. Hinges are spaced every 6" or around 5 per side for my meter spanned wing. On your wing, add some hinges. I use and really like the Radio South CA types, but there are others that work well too
Oh yeah almost forgot retracts holes and servo cut outs. I tend to not use anything extra here in my 2 meter models but on your jet, you may want to add 2 oz cloth on the skins before veneering, with considerable overlap of 3-4" all around
Good luck. It sounds like a real fun project....