[4 L-bodies]

Concerning your low speed poor performance, are you sure it is dropping down into first gear from a stop? I have the same car and mine accelerates fine. Not as stout as a stock GLH-T, but acceptable. Sounds like your starting off in second gear not first. It should be shifting out of first gear around 13-15 MPH and into drive around 25-30 MPH under normal (light) throttle loads. Perhaps the governor in the tranny is not working properly?
Cool about the original owner. Sounds like a guy I'd love to talk to. Always had a soft spot for the Mercury cars esp. the performance models like the Eliminators, GTE's, XR7-G, and Comets too. Way back in the day a short order cook at a Mr. Steak in my hometown drove a GTE. Black with silver trim. Unfortunately he drove it year around through the snow etc. Tried a couple times to pry that car away from him, but to no avail. I always knew how special that car was, but apparently the owner did not. He was just using it for transportation, and this was back during gas crises around 1975.

 

[4 L-bodies]

Since your tranny is starting out in first gear, I'd be checking cam timing as next likely suspect for very poor low speed performance. Retarded cam timing one or two teeth could cause symptoms you describe. Low power at low RPM, then have normal power at higher RPM. While your at it also check ignition timing too.
Like I said, I have the exact same car. Friends that drive the car comment on how much power it has.

 

[4 L-bodies]

Well your making progress. Yay! I once bought a 50K mile GLHT and it took me 3 months to fix all the gremlins. Sat for 15-16 years. Wouldn't start, wouldn't stop, and it wouldn't shift. My thought was how bad can a 50k mile GLHT be? Had some buyer's remorse, but it just took some time to sort out.
Rear wheel cylinders are probably frozen and non functional. They will still bleed okay, but cylinders won't actuate. Either that or parking brake cable(s) are frozen. I've had to replace both on just about every drum brake TD I own.
As far as the TB throttle stop. It should just be a 8mm nut to loosen on back side of TB, then a small flat blade screwdriver to adjust idle speed. This of course is a temporary fix until you get the AIS motor issue fixed.
If you don't have a timing light, adjust for highest HG vacuum reading, then retard a bit until vacuum just starts to drop. that should get you close. Should have around 18-20 HG at idle.
Don't get discouraged. I was there too when I bought my NY'er about 10 years ago. Been very reliable ever since. Just need to get the bugs worked out.