First, I acknowledge that unlike some other projects in Post Jam Jam, DuskMiner has not been worked on as long and so it is quite impressive how much you were able to implement in such a short period of time. So good job finishing the jam and good job getting in as many features as you did. That said, in the spirit of giving constructive feedback to help each other improve, I am going to be very honest here.
* How does combat feel?
Combat was more frustrating than fun. Having LMB be both "walk to" and "attack" was frustrating because the game often walked somewhere when I wanted to attack and attack when I wanted to walk to somewhere. It's nice that you implemented ranged, melee, and tech combat, but there wasn't anything particularly interesting about the combat that would make me want to continue engaging in it. Also, I only encountered 2-3 enemies each playthrough so there wasn't much to base my combat impressions on.
* What is your impresson of online play?
I wasn't aware that it is an online game. I just tried it right now by launching the game in 2 separate browsers and it worked. I can definitely see how online play makes the game more fun and interesting. Combat is more dynamic when you can attack as a team and use different weapons.
* Would the game represent well as mech-based combat?
The game does not currently represent as a mech-based combat game (at least visually). If you're asking if it could be a mech-based combat game I don't see why not.
* What would it take to make it 2v2 or 4v4 or any other small team deathmatch?
I was under the impression this is more of a co-op, PvE action RPG game. If you wanted it to be a team deathmatch game I guess you would need a scoring system, good arena level design, perhaps power-ups?
* Do any of these new features sound interesting?
Heat management and salvaging defeated enemies sounds like it would add depth to the gameplay.
Concluding thoughts: It seems like the game doesn't know what it wants to be and from your questions, it seems like you don't know what you want the game to be either. My advice would be stop development and start designing. Write a Game Design Document, figure out what you want the game to be on paper before you put in countless hours developing it. Right now it seems like you're going about it backwards and trying to design the game as you develop it which is going to lead to a lot of wasted time and the lack of clear direction showing up in the game itself. I hope this feedback helps and good luck to you!