1) Bottom stacklets are fixed size. Stack overflow (which can happen on the very first frame if it’s big enough) pushes a grant of the occupied part of the overflowing segment, allocates a bigger (policy) stack segment somewhere, and sets the various specRegs to describe it and thereby grant permission. Return unwinds. Unwind is lazy so you don’t get ping-ponging segments if you are real close to a segment boundary.
2) I doubt that the search hardware would be exposed. To specialized, and will vary by member so not portable.
3) I looked at the exokernel work. So far as I can see “exokernel” is just marketese for “microkernel” plus libraries. The libs would be services on a Mill, but otherwise I haven’t seen anything that novel compared to prior work in microkernels and capability architectures. Please point out anything I have missed.