Not sure about question 1 as i dont have a TD4 to look at. But, questions 2...You need a tamper proof torx bit. Cheap enough from Halfrauds or other such motor factors.
The Hippo drives the front wheels faster than the rear wheels. The VCU slips in order to compensate for this. If the VCU fails, the prop will put stress on the IRD and cause it to fail also.
The blanking part is not essential, no. It is normal to feel some resistance when on full lock. There is no problem at all with leaving it in 2wd. Just make sure you inform your insurance company.
Try cleaning the MAP sensor on the back of the inlet manifold. These can get clogged with carbon
EDIT: Just seen you have replaced the MAP sensor already, sorry