Hi,
Just a few observations on your points based on my recent purchase experience:
a) sales man would always want to avoid a PDI and that is simply because it is an additional hassle for them (and another point of contact where customer can change mind
)
The only way out is to be stern in the request and not make it sound like a request. It is your right to inspect the vehicle and sales guys are quick to notice this if we ask it.
b) For most of the dealers they have executives over the sales persons ( sales manager, AGMs...etc.) who can be contacted if required.
c) I would personally suggest not to miss on the PDI for any leverage. A couple '000s are not worth risking the quality of car you are about to get. Moreover, for most cases no company has such strict policies(enforced) for the sales process. Most of the dealers have their leeway for the end sales process.
d) for specific case of decoding the VIN, before proceeding for registration, the least that could be done is to ask for Form 22 (not sure of number though). This document has the date of car manufacture.