Is it the same when retarding to idle in the flare - that is, you have to pull the levers firmly back, against the resistance of the 'detent' mechanism, to make sure that they are firmly seated?
No.
Idle has no detent. It is merely a stop, although it is sometimes referred to as a detent, even in the FCOM. Look at the schematics posted here a while back of the entire thrust lever mechanism. The detents also have no resistance getting into, only getting out of.
The non-detent nature of the IDLE stop is also the reason for the observed behaviour coming out of reverse. Going slightly beyond (forward) idle, and pulling the levers firmly back to the stop. This has also been quoted from some training procedures quite early in the thread.
The reason for pulling the levers back quickly is to cut thrust quickly to avoid a prolonged float before touchdown.
Bernd