Why Mugen MBX5T kit designed accept only 150mm long bats?
 

That limitation is frustrating. One reason do not buy this kit...

PolyQuest and some NEU's are 165mm long and they and impossible to fix them to KIT
One simple solution to turn servo 90 degrees and it will give exact space for longer bats. What you all think abot this?

The batteries you listed will most likely fit in the MBX5T kits, some without any mods. The trays will fit batteries up to 160mm pretty easily. Also, for anything bigger, you can just grind down the back lip of the tray to fit a longer pack. The three straps are what is holding the battery to the car, not so much the tray itself.
As for the ideas you listed, turning the servo would work except that you would then need a special servo horn to make it work, like the Hot Bodies D8 uses. Also, the battery tray would have to be moved away from the center of the car so that the battery could safely clear the center spur which would unbalance the car.

Yeah, means that you need to grind and hack it. I never seen kit in real life so my suggestions based on pics. But clearly with eye i see that there are pleny of space for servo turning like on HB D8.
But honestly I think that KIT is more unbalanced right now all weight is concentrated on back of the car.
2/3 batt and 3/4 motor weight + ESC + RX box.

Lets say ~350g of batt, ~300g motor, 150g ESC, 100g box+RX.~900g back
150g batt, 2 servos 150g, 150g diff +100g clutch and brake stuff ~550g front

I am sure that flips in air is difficult to do with this KIT
So moving batt to more further will help balance weight, and protect against spur is not the problem at all.