I think both of the NGV and the tunnel are stationary parts that will not affect the rotor preload operation. The tunnel will not move whatever the NGV is tight or not. It could induce a vibration on the rear shaft if there was a bit of free play on the rear of the tunnel because the NGV is also the rear support of the shaft.
To remain the NGV at the plane is true vertical with the axis of the rotor is a key factor to keep the turbine from contact.
I have an other idea to reduce heat on the rear bearing. Since there is a gap between the platform of turbine and the inner platform of NGV, so hot gas can intrude into the rear bearing area. We can drill some holes on the inner flange of NGV to let cooler air around the CC to push the hot gas back its original path. The cooler air has higher pressure than the gas from the NGV. Any comments about the issue?