Yes. If you are a rated pilot and get solo endorsement in another category that does not have a specific expiration written in the solo endorsement. No passenger carrying but the XC and other student limits do not apply. No FR is ever required. Mostly pointless but if you had a single seat helicopter or sailplane you could do it. Most transitions are from airplane to other so other category CFIs are more likely to be aware and not sign you off for infinity. A friend flew sailplanes for many years on a solo endorsement eventually he purchased one and the insurance company required him to get his rating within a year.
No. Once you give him a 61.31(d)(2) solo endorsement, he is free to fly an airplane solo anywhere he wants, any time he wants, within the limits of his Private Pilot privileges unless you put additional limitations in that endorsement.
No. He would still need a current flight review. The exception in 61.56 for Student Pilots in 61.56(g) does not apply to other pilots flying under a 61.31(d)(2) endorsement.
You keep saying this, but it is not true. There is no exception in 61.56 for non-Student Pilots to act as PIC without a flight review (or equivalent) while flying solo. The exception in 61.56(g) applies only to Student Pilots.
