Cause: Strider has a turret while the Irfit and Hunter used by the other factions don't have turrets. Therefore you get a group of 5 units not 4 as intended when using BIS_fnc_spawnGroup:

several functions are called with BIS_fnc_spawnGroup:

BIS_fnc_spawnVehicle
BIS_fnc_spawnCrew.sqf
//Search through all turrets and spawn crew for these as well.<=== extra crew member spawned as it searches turrets

> BIS_fnc_returnVehicleTurrets;

_grpName = [(player modelToWorld [0,20,0]), resistance, (configfile >> "CfgGroups" >> "Indep" >> "IND_F" >> "Motorized" >> "HAF_MotInf_GMGTeam")] call BIS_fnc_spawnGroup;

Actual Behaviour - see config entry below:
unit0 "I_soldier_F" - Driver
unit0b "I_soldier_F" <======spawns extra unit in commander turret
unit1 "I_Soldier_TL_F" - on foot
unit2 "I_support_GMG_F" - on foot
unit3 "I_support_AMG_F" - ****no room in vehicle for this guy

Temporary fix - delete the extra soldier in the turret or the driver?:
_vehicleName = vehicle (leader _groupName);
_unitName = _vehicleName turretUnit [0]; moveOut _unitName; deleteVehicle _unitName;

Possible Solution:
So you need to delete the Team Leader from the cfgGroups entry to get the right number as there is already a commander in the vehicle?:

~~~~~~~~~~~~~~~~~~~~~~~~~
class CfgGroups {
~~~~~~~~~~~~~~~~~~~~~~~~
class Indep {
name = "Independent";
~~~~~~~~~~~~~~~~~~~~~~~~
class Motorized {
~~~~~~~~~~~~~~~~~~~~~~~~
class HAF_MotInf_GMGTeam
{
name = "Motorized GMG Team";
side = 2;
faction = "IND_F";

class Unit0 {
side = 2;
vehicle = "I_MRAP_03_F";
rank = "SERGEANT";
position[] = {0,0,0};
};
class Unit1 { <============ Delete this entry so enough seats for team - the commander that spawns in the vehicle is assigned as group leader by default.
side = 2;

	 vehicle = "I_soldier_TL_F";

rank = "SERGEANT";

	 position[] = {5,0,0};

};
class Unit2 {
side = 2;

 	vehicle = "I_support_GMG_F";
	 rank = "CORPORAL";
 	position[] = {-5,-5,0};

};
class Unit3 {
side = 2;
vehicle = "I_support_AMG_F";
rank = "PRIVATE";
position[] = {5,-5,0};
};

You would need to do the same or check all Independent motorized classes that use the "I_MRAP_03_F" Strider:
"HAF_MotInf_Team" <=== OK 3 units
"HAF_MotInf_AT" <=== ! 4 units
"HAF_MotInf_AA" <=== ! 4 units
"OIA_MotInf_GMGTeam" <=== ! 4 units
"OIA_MotInf_MGTeam" <=== ! 4 units
"OIA_MotInf_MortTeam" <=== ! 4 units

Confirm - this occurs with
{_x setposATL [getposATL _x select 0, getposATL _x select 1, 1000];} forEach units group player;
and in a helicopter cargo position:
{_x action ["eject", vehicle _x]} forEach units group player;
So 'throw' AI in the air and ejecting AI doesn't work. AI doesn't have HALO an nimation. They are separate from the parachute and AI dies upon landing.

To equip a backpack it's:
this addBackpack "B_Parachute";

It's standard in the US Navy / Marines to use an Emergency Breathing System (EBS) to exit ditched helicopters. NATO were looking at making it standard and it's is standard in the North Sea oil industry. Examples / training videos here:
http://ftp.rta.nato.int/public//PubFullText/RTO/AG/RTO-AG-HFM-152///AG-HFM-152-07.pdf
http://www.youtube.com/watch?feature=player_embedded&v=K9jn-POo6W0#!
http://www.youtube.com/watch?v=SysFQ-iCGSU
http://www.helicopterhelmet.com/HEEDS-III-Aqualung-SEA_c_18.html

You can't add them via script either, only in the inventory in game:

The related other half of this problem here:
0005526: There are no functions for adding weapons to a player's vest or uniform (unitVest, and unitUniform needed)
http://feedback.arma3.com/view.php?id=5526

Something else I thought of - it's 2035 - all these small UAV's we are warned about may be common place. Is this what is being simulated at the higher difficulty settings? Are the AI getting pictures of your location on those helmet visors? The battle simulation at the level of current technology is off as you say - but is it correct for 2035?

It would be nice if BIS could say exactly what the future vision is as it may have some impact on this debate?

What skill / difficulty settings did you use for that? They only seem to do it with higher settings? If you lower the skill / difficulty they seem less sure where you are?

Try lower skill settings with the setSkill array cmd on these one at a time and then compare it with the 4 different difficulty settings to identify what's causing it?

"spotDistance","spotTime","commanding","general"

This causes major problem with saving a loadout and adding it later upon respawn. The player can equip weapons to a uniform / vest from an ammo box, but you can't add them using a script? So saved loadouts don't work properly?

unitVest / unitUniform would be great commands to have so the containers can be as useful as backpacks.

Missed out some steps above - you need a MH-9 too:
6b. place MH-9 with crew
6c. sync MH-9 to Module: Support Provider: Helicopter Transport

Why don't you just decrease the AI skill to 0.3? There is a slider in the editor, script commands and even a new module that you sync with the group leader? Lower the AI skill and they stop being super human.

Not sure what causes this but 25% of the time the M8 SLAM detonates when you place it and kills the player. I have decided to stop using the thing as it's so unpredictable.

I can confirm this - when you get to surface bubbles are out of water in the air above you and move up and down in time to sea/player moving on sea. When I went on land they persisted. It looks like a particle effect attached to player.

edit: possible something has changed or it only affects standard/low graphics settings - saw it the other day when trying to get more FPS - can't repro it now with settings on very high.