1) Execute must be used with Ambush, so the total Stress cost to "Remove all white dice from the target's defense pool" (and gain a "Pierce 2" even if the target has no defense dice) is 2 Strain, right?

2) After using Target Acquired (to get LOS through other figures), you can't use No Escape to "perform 1 additional attack targeting the same figure", because for that second attack against the same figure, other figures now block again LOS, right?

I think you are correct on both points.

1) says nothing about replacing Ambush

2) yep. thats a second attack.

Regarding Ambush:

So it works w/ Mak's invisibility at 4 and above spaces...

But what if he and his desired target are both out of line of sight, like around a corner. Mak declares his target - who cannot see him - then moves out into line of sight w/ 2 pierce??