present perfect versus simple past

A

I think both are ok. But we'd be more likely to say "where we left off". It doesn't change the meaning of "where we have left off" - actually we'd say "where we've left off" rather than "we have", but in English, if a word can be omitted, then it's good practice to omit it.

we have left off means not too far from now in the past

left off is better here, IMO.

The present perfect is not used with a definite point in time, even if that point in time is not explicitly stated.

After lunch I want us to take up the discussion at the point where we were at the time [we left off / *we have left off].

Implicit statement within the larger one:

We left off at that time.
*We have left off at that time.

C

thank you Jim.