As far as I know, the 4th protection module slot never goes on sale. It has always been at full price ever since it was added. :(

It is because...
1. One of your teammates is using the "Engineer Drone". Whenever they pick up a supply box, all of their allies on the map (including you) will also get the same supply activated for free :)
2. One of your teammates is using the "Supplier Drone". Whenever they activate a double damage, double armor, or speed boost while they are near you, the same supply will also be activated on yourself for free :)
3. One of your teammates is using the "Mechanic Drone". Whenever they use a repair kit while they are near you, the repair kit will also activate on yourself and heal you for free :)
4. A teammate using Dictator uses overdrive while they are near you, and the effect also activates all your supplies for free :)

Yes, it decreases based on how much time is left. The longer you wait, the cheaper it gets.

FunFact: When there's only about 3 seconds or so left on a Micro-Upgrade, the Speed-Up will actually become free (it will cost 0 crystals).

I estimate about 30 people who "regularly" use the forum lol.
And another 100 or so that only post once a month or whenever a new update comes out.
 
That's not counting the truly inactive users.

It's probably just someone who made an account just to carefully farm goldboxes and upgrade equipment while staying at the recruit rank.

You will always see your drone and the enemy drones above the tanks unless.....
The drone is on cooldown/recharging. You disabled the drone visualizer in the settings menu.

Yes, because the Incendiary Band alt does not light tanks on fire when you are overheating.

It lights tanks on fire when your ammo bar runs out.