Gosh Car Ramrod, you sure are focussed on theoretical safety issues. You should apply for a job in the regulator.
Would you, regardless of the new rule, when part way through your flight upon realising you aren't going to make your destination without eating into your FR, divert somewhere if you have the ability to?
It depends.
Well, it may be just as safe, or safer, to continue to the original destination and land with only .... deep breaths now, because this could be confronting ... only 40 minutes of reserve. (My profound apologies to readers with delicate constitutions.)
[W]ould you declare an emergency[?]
Definitely not. The circumstances do not satisfy the definition of "emergency".
[H]ow will atc know you are fuel critical and if they vector you around you could run out?
I'm not "fuel critical" and ATC always know I could run out. That's the consequence of being in the air in a powered aircraft with finite fuel capacity. If being under 45 minutes' FR is a "fuel critical" "emergency", how could it possibly be acceptable to allow people to fly around, today, and land with less than that? Surely some kind of AD or grounding action is required?
Car and actus: you'd do yourselves a favour if you researched the cause of accidents that result from no motion lotion getting to the thrust generator. Knowing the cause helps to design systems to reduce the risk of it happening.