When has a pilot legally accepted an ATC clearance or instruction?
I would try to not over complicate this issue.  There are three conditions a clearance/instruction can have:


Transmitted/Received
Accepted
Rejected


If ATC provides a clearance/instruction, but has not yet received a response, the clearance/instruction has not been accepted or rejected.

If the pilot acknowledges the clearance/instruction with a read-back or affirmation of understanding, then it is accepted.

If the pilot responds with the term "unable" to any element of the clearance/instruction, then the entire clearance/instruction set is rejected, which may prompt ATC to inquire as to why the clearance/instruction is being rejected or to provide a different clearance/instruction.

Per 91.3 the pilot-in-command reserves the right to act as the final authority as to the operation of the aircraft, but once a clearance/instruction is accepted, it is an indication that the pilot-in-command is agreeing, as the authority of operation, to operate by that clearance/instruction until released from it or forced to deviate due to an emergency or TCAS alert.

One note for safety: to be clear, one cannot simply reject one element of a clearance/instruction without rejecting the entire clearance/instruction.  For example, if ATC issues a turn and a climb, but the pilot's aircraft cannot or may not climb higher for whatever reason, the pilot should not read back the new heading and say, "unable climb," and then turn to the heading and wait for more instructions.  What if the climb is to avoid obstructions that exist on the new heading?