The contracts clause doesn't appear to apply to NPVIC. Rather, the 3rd paragraph of Ariticle 1, Section 10, does directly.
In any event, there is no such thing as settled law as long as economic, social, and political landscapes continue to change along with the justices occupying chairs on SCOTUS.
I expect NPVIC will be challenged and the case will be heard by SCOTUS. I further expect that it will be found unconstitutional.
As to how many people would I be comfortable disenfranchising, at least 3 million as an established, tested lower limit. (I sleep well at night.) I'm not sure about the upper limit.
As to the ends justifying the means, I don't see that applying in the case of NPVIC. You are attempting to tamper with and alter a constitutional provision based on what you perceive to be a noble goal. The magnitude of the change you seek demands a significantly higher level of discourse, debate, and approval than that provided in the NPVIC movement.