The text on the Mark V Sentinel for Boost says to put two +1ATK/+1DEF counters on it. So, does that mean that it retains its +2ATK/+2DEF ability (i.e., the counters) into the following turns? Even if stunned and then recovered?

The way I read it I think so...can anyone clarify?

yes, it retains its atk and def counters throughout the turns, even if it gets stunned.