If I use command to interrupt and give an attack order to another hero, can they take strain to use their abilities on that attack?

Can they exhaust cards on that attack?

Or is it just their base attack from a weapon they have equipped?

For instance, can Gideon order mak to attack, and then mak take a strain to get the pierce from his Ambush ability?

I assume "yes" as long it's not precise "during your activation"

If I use command to interrupt and give an attack order to another hero, can they take strain to use their abilities on that attack?

Can they exhaust cards on that attack?

Or is it just their base attack from a weapon they have equipped?

For instance, can Gideon order mak to attack, and then mak take a strain to get the pierce from his Ambush ability?

Cards that say "exhaust when declaring/performing/resolving an attack" would be fine, since the other hero is indeed performing an attack under Gideon's orders. If that card also has a strain cost, that would also be fine I think.

However, the hero would not be allowed to take strain for movement; RRG page 23, "Strain" says "A hero can suffer 1 (strain) at any point during his activation to gain one movement point. He may do this up to twice per activation." The key word is "his", and your example, it would be Gideon's activation and not Mak's, so Mak would not be allowed to take strain to move.

Cool, but he could strain to activate ambush and thus get two Pierce it sounds like.