"Then I pulled the return line off the fuel rail, turned the key (to position II) and saw a *surprisingly* large gush of gas squirt out..."

That was the pressure line at the fuel rail. The return line comes from the FP Reg. The "gush" was probably from the Key On buzz of the Main pump. Or maybe just from the Tank pump.

In your original post you said:
"- Key-Position II produces the usual 'buzz' from the rear of the car & from under the hood as well." Did you mean under the "car" rather than under the "hood"? Because the Main pump is under the car.

My hunch, given "good spark" at the plugs, is that you don't really have good pressure at the injectors. Sometimes an older pump will stop on a dead spot or something, and will come to life only when tapped (a light smack) with something solid.

But before you smack the Main pump, take a minute to test each pump individually...
Remove Fuse 4 and jumper +12V from the left side of any fuse 6-10 to the Fuse 4 LEFT contact. That should power the Main pump by itself. Then put the +12V to the Fuse 4 RIGHT side contact to power just the Tank pump.

Also, pull a plug or two after cranking to see if wet from fuel.


--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.