a) If a towered airport operating nothing more than an Aerodrome Information Service were to say to a joining aircraft ,"Report Downwind", does the "instruction" have to be read back, or does "Wilco" suffice?
I would suggest that there are no circumstances in which a
reporting instruction should be read back.
ATC can never safely assume that a report has been made as requested, as there is always the possibility of comms failure. Thus a report can enable a clearance that must be witheld until the report is made (e.g. a procedural separation: "G-AB passing 4000 ft in the climb", "Roger G-AB, break, G-CD climb to altitude 3000 ft"), but its absence can never trigger a critical situation (e.g. if G-AB never reports, G-CD just stays at 2000 ft).
The disaster scenario is where a report is made (or is misinterpreted as having been made) before it should be. Consider:
"G-AB report passing 4000 ft in the climb"
"[snap, crackle, pop] Passing 4000 ft in the climb, G-AB"
"Ah, roger G-AB, break, G-CD climb to altitude 3000 ft"
In fact G-AB is still at crawling through 2300 ft, and was just reading back the reporting instruction. He's about to be hit by G-CD...
Overall, safety wins if reporting instructions are
not read back.