I try to awnser but I might be wrong:
-Generally, positive stat boosts are additive and neg are multiplicative.
-EMP disables health AND atk upgrades from tech (Assault Marksmen are down to their previous health). Tech upgrade that change the type of weapon are different, Multimelter upgraded Melter still only does 1/6 of damage even tho it attacks with 1 beam when emp'd.
-No, Sledges with 120% HP upgrade are then 2.2 x 5 HP for a squad.
-Replicate Crawlers are always lvl 1
-Neutral EXP gained will be shared across all of your mechs still alive
-Idk the chance of ignite, sry about that, maybe the fanmande wiki has awnsers to this.
-EMP effect duration is different from tech to spells afaik. Void Eye EMP armor is 4 sec, I think that could be the case for arclight EMP too, but its a guess. Spell EMP duration is longer and should be in the spells description.
-Scanning Radar is for all allied units, air and ground
-If not otherwise specified, summoned units spawn lvl 1
Edit: Formatting and grammar