Apologies for the delay in following up here. Fuel pressure regulator tested, and functioning appropriately.
I have shifted my focus toward electrical items, as I opted to run some basic powers/grounds and ref voltage checks and found a power feed to the ECU lacking any voltage whatsoever (coming from the junction block). I've also got a couple 5V ref outputs measuring slightly lower than they should be. New strategy will be to remedy the failed 12V power to the ECU, and then check whether the 5V ref circuits are remedied or not.
Interestingly, I get no tach signal to the dash, I have an intermittent ELC-4 and CEL light and obviously no start... One of the 5V ref outputs measuring lower than it should (3.48V) is port 51 (white wire), which feed the cluster (tach and CEL), ELC control module, and ignition power transistor.
No closure yet, but now have a clear next couple steps. About to get snowed in for a couple days up here, but will follow-up once I've followed this lead.