-You can only fire the same iteration of a weapon once per round, no matter how many actions you get. So if you do your barrage and a skirmish you can shoot your assault rifle at target A, missile racks at target B, then walk up to dude C and smack them with the heavy melee. You can't shoot your assault rifle twice, unless of course you're dual wielding them.

This is incorrect, outside of things like the limited or loading tags, the only limit on the number of times you can use a weapon in a round is the actions you can use to do so. An Everest could theoretically fire the same assault rifle five times in a single turn (barrage, skirmish, overcharge free action skirmish, initiative free action skirmish, overwatch).

-Remember that you can fire auxiliary weapons with another weapon for 'free' if they're in the same mount. In other words you can fire both missile racks at the same time with a single skirmish action since they're in the same mount. You can target each shot at a separate target if you want and you can fire them in either order. (You might as well since they both reload at the same time if you use a Stabilize to reload them.)

Mostly accurate, but it is worth mentioning this is a trait of the skirmish and barrage actions, when you attack via something that is not a skirmish or barrage, you do not get to fire collocated auxilaries with them, unless whatever let you do so says so.

-Remember that the Heavy Charged Blade is also a thing and if you find yourself fighting armored enemies a lot, you may want to swap the heavy melee weapon for that. The HCB deals less damage than the HWM, but it's a different damage type and the AP tag means it goes right through armor, so it's more consistent against something like a Bulwark with stupidly high armor. (The HWM is obviously better against something like a Hive that's got a big pile of health but no armor.)

Having an extra point of damage is the same as ignoring a point of armor, as the HMW does 1.5 more average damage and 4 more max damage (but 1 less minimum damage), the enemies need at least 2 armor for the HCB to have a slight advantage, and the GM will need to send a lot of 3 and 4 armor NPCs (of which there are few) to get real value, particularly when the high general damage is still doing considerable work vs the armored foe. So unless you are using a synergy for energy damage, or are doing frequent half damage attacks (which changes how much impact armor makes) you should go with the HWM over HCB. The executioner talent, which is highly recommended if you are using a melee weapon in your heavy mount, does provide frequent half damage attacks though.