A YV-666 piloted by Bossk allows you to cancel a critical hit to add two regular hits. What is the order of resolution with this? Can I use Mercenary Copilot to turn a regular hit into a critical hit, then turn into two more regular hits?

What cannon would you recommend for this, Autocannon or Mangler Cannon? I realize Mangler guarantees you a critical hit, but if Mercenary Copilot stacks the way I think it will than would Autocannon be a better choice?

Mercenary Copilot is Range 3 only and Autoblaster is Range 1 only, so would not work.

Mangler Cannon is Range 3 so would work. At Range 3, you could use both Mangler and Merc to get 2 crits and then convert one to 2 hits.

If you rolled 3 crits, you could use Bossk to convert the crit to 2 hits, and then Mangler cannon and Merc to make it 4 crits total.

Bossk's pilot ability doesn't trigger until the attack has hit. An attack hasn't hit until the end of the Compare Results step. Autoblaster and Bossk do not work well together, because any crits you roll with the Autoblaster can be canceled by evade results before Bossk has the chance to change one into two hits.

Basically, if Bossk has two hits and a crit and the defender has three evades, you can't use Bossk to force one damage through. If Bossk has two hits and a crit and the defender has two evades, you can decide whether you'd rather deal one critical damage or two regular damage. That's what Bossk is for: all those times when you get one crit through but it spangs harmlessly off the shields, or you're hoping for a Direct Hit because it'll finish the target off, but you get a Blinded Pilot or something instead.

Mercenary Copilot is Range 3 only and Autoblaster is Range 1 only, so would not work. - Correct

Mangler Cannon is Range 3 so would work. At Range 3, you could use both Mangler and Merc to get 2 crits and then convert one to 2 hits. - Partially correct.

If you rolled 3 crits, you could use Bossk to convert the crit to 2 hits, and then Mangler cannon and Merc to make it 4 crits total. - This is where the order is incorrect.

Follow this:

1. Roll attack dice (crit, crit, crit - lucky roll!)
2. Modify attack dice (Mangler and Merc here, but they've got nothing to convert, so neither of them come into play this time)
3. Roll defense dice (evade, evade)
4. Modify (none for the sake of this example)
5. Compare Results (crit, crit - one crit is uncancelled, so Bossk has 'hit', thus triggering his ability now, and he can convert the crit to two hits)
6. Deal Damage (two facedown damage cards to the defender)

Bossk's timing is very specific and only happens after the Compare Results step and ONLY if he scores an uncancelled crit.

You can heavy laser cannon too, with merc copilot. The crits to hits part of HLC only refers to the initial roll and not any mods / rerolls.

Bossk still has to 'hit', and the difference between 4 dice and 3 dice is more than 1.