Why? Speech doesn't cost anything. I think it'd be safe to assume that a headset is used. Agreed on other points though 
Because you are basicly calling for aid in a battle in form of other players, you could compare it to the use of Fa or whatever. I agree it should defiently cost quite some ap.
5-6 should be fair, you are calling in reinforcements after all, which easily changes the tide of battle.
The best solution would be imo to just have the option to switch between RT and TB during battles
Like a voting system, if half of the combatants vote yes, the game should close combat and RT starts
Only if it would have a timer, or even beter if it requires 2-3 full rounds before it takes effect. (so you can't just take long turns so the timer can run out)
Otherwise you would get this situation:
Group A encounters loner B
Loner B decides to run away in TB
Group A sees that B is going to get away, and quickly uses the voting system to switch to RT (They outnumber B after all so B can't do anything about it)
Loner B ends up with 5 bullets in his head because he couldn't leave the map duo to the RT combat in effect.
