For your first question, it's an "inline if" or conditional expression, it goes like this "logical expression ? true expression : false expression". In this case, I multiply the width by -1 if mirrored, or by 1 if not.
Yep, indeed, it only works with static object because I used a static number in the formula. Give it another go, I changed the formula a bit so it now uses a percentage of the original animation frame width.