there should be a button for each of the buffs/object interactions. but you dont apply/interact by pressing the button, you just select a “tool”. the actual buff/tool is applied/used by “left mouse button”. self buff is applied with “right mouse button”.
example:
so you have buttons “1-4” to select class specific special abilities. (healthbuff, kevlar armor, adrenaline, buff weapon, speed buff, metabolism, give ammo… … …) you equip a buff tool (very high, almost instant equip speed), so when you apply the buff with “left mouse button” you dont fire your gun by accident.
with “q” you select “syringe” for medic, “bomb” for soldiers , “welding torch” for engies, “hacking device” for operatives. and use them with “left mouse button”.
“use” key can now be used for command post interaction, machine gun nests and the first objective of security tower, where you have to open the small gate before planting the bomb.
edit: ok. thinking about it, the thing with the left mouse button might be a problem. solution would be similar to the suggestion above. except, exchange left mouse button with “use” key.
so, you select your buff tool by buttons 1-4, hit “use” to apply for friendly/self. if no buff tool is selected, the “use” key is for interacting with command posts, gun nests, sec tower obj. “q” would select your primary class ability tool (syringe, bomb, welding torch, hacking device), “use” button would execute them.
basicly its just like how the special grenades work. you select (for example) the molotov with one key and actually use it with another. thats the whole concept.