1. Q1: Does gas return to it’s original state a - b - c ?
No, the rate of compression/expansion matters.

2. Q2: Does gas ever return to it’s original state
a - b - c - d ?
Yes ! (and the step c - d is very simple).

3. Q3: Does the universe (gas + reservoir) return
to it’s original state a - b - c - d ?
No ! (we changed the state of the reservoir in
the steps a - b and c - d).

4. Q4: Does the universe (gas + reservoir) ever return
to it’s original state a - b - c - d - e?
Sadly, no. (the experiment is intrinsically
irreversible).