Am I understanding that this is over radiant heat in a basement? Solid? Only engineered is acceptable with those conditions, and even then, with low expectations as to performance.

No the oak will go on first floor. Radiant is in basement.

I have heard this recommended for wide width solid flooring in Vermont, for instance. I have never glued solid down, except in a tight closet.

My experience tells me that radiant heated houses provide ideal conditions for building materials to 'live' in and wood flooring remains flat and moves very little. I don't know what others might say about this, but my initial reaction is the floor would be fine without the adhesive.

I don't have experience with such, but it seems that it might be more efficient to T&G the ends, rather than biscuit them. A little easier to cut the biscuit slots, but then you have to fiddle with it during installation.

We have already done the "biscuiting" on all the ends.
We have decided that we will not glue oak to the subfloor.
This is 5 1/2" wide 3/4" solid oak. I guess it will move some, but doubt glue is the an appropriate remedy in our house.

Many years ago I installed a Bruce oak floor for my sister and BIL, prefinished with a wax finish, random width like 3,5, &7 inches. The instructions told me to glue (liquid nails) under the tongue edges of every course, which I did, and they never had any cupping or squeaks in that floor, even though their builder (five year old house) was a real jack-leg and the subfloor was CDX ply -- really rough condition. It was nailed with regular floor cleats. FWIW.
Casey