a. Has any thread, excluding quizes etc had so many posts?
Yes, I remember thinking about this (maybe it was discussed?) a couple hundred posts ago.
The Non-urgent TOPS requests thread is at 31k (urgent is at 18k),
A career as a signaller has 19k,
First Greater Glasgow (buses) is at 14k,
Lothian Group (buses) is at 13k,
The general discussion thread on football has 33k.
The long lasting "Northern Unit Refurbishments" thread has 10.5k, but this will likely overtake soon enough.
Only scrolled through threads posted today/late yesterday, there's probably some higher numbers around.
Plenty of rolling stock threads have large amounts of posts, the 755 thread has just over 8k