HLC turns any crits to hits, and Etahn turns 1 hit to a crit. How would these two resolve?

HLC says "immediately turn all crits to hits" so it happens first, and only after the initial dice roll. Then you are free to modify with other abilities like Etahn or Mercenary Copilot, etc.

Edit: also, any crits from rerolled dice from the HLC attack do not get turned to hits, so if you use a TL or Howlrunner to reroll dice, and they come up as crits, they stay crits.

HLC is applied immediately after the initial roll, before TL or marksmanship or Etahn or whatever else is applied.

Like the Cpt says, it's only the initial roll that gets changed. All other crits remain and other abilities can change hits to crits. There is even a FAQ entry for it:

HLC says "immediately turn all crits to hits" so it happens first,

The word "immediately" actually isn't the relevant term. Heavy Laser Cannon says, "Immediately after rolling your attack dice, you must change all of your [big explosion] results to [small explosion] results." The italicized text refers to the "Roll Attack Dice" step of the attack sequence, which precedes the "Modify Attack Dice" step. After you change the crit results to hit results, your opponent has the opportunity to modify the attack (using Sensor Jammer or R7, for example), then you can modify the attack using Etahn or any other offensive modification ability.

The card text is not especially clear but as Hexis mentioned, we know it works this way because of the FAQ.

I want to try Ethan plus two blues with HLC. You got 10 points of upgrades to play with. Probably R2-D2 and Adv. Sen on Ethan.

Edit: also, any crits from rerolled dice from the HLC attack do not get turned to hits, so if you use a TL or Howlrunner to reroll dice, and they come up as crits, they stay crits.

Except Howlrunner only effects primary weapon attacks. I believe Jonus is who you were looking for.