Distributor advance springs

I’ve gotten the distributor all the way apart and pried the weights away with my fingers and they snap right back to center. Certainly not the issue. At least, not on the 77 distributor.
If you want to better understand how the springs and weights work, pry on just the weight attached to the blue secondary spring. It has a long loop, so that spring only engages after the the weights have moved out X number of degrees. The primary spring applies alone applies force from zero rpm until the secondary spring engages. The weights are joined by the slotted plate. This was how Chrysler shaped the advance, providing a quick advance off of idle, then a slower advance.

Now I’m beginning to believe that the issue is that the timing was set with a low idle.
Nope. Initial should always be set at slow idle. Did you read the snips from Chrysler Tech that Bugman posted in the thread I linked earlier?

You can't set initial timing at an rpm where the weights have begun to advance.
Lets use a '67 distributor as an example.
Take your pencil and put it on 5* BTDC but at 1200 rpm instead of 600 rpm.
At 1200 rpm the timing is the initial plus 5* of advance from the weights moving outward.
Timing at all rpms will be late, power and efficiency will be lost. That lost energy will go into the lower cylinder walls and exhaust as heat.

He told me it sounded a lot like weights stuck out. I asked if maybe it wasn’t that the weights were stuck out, but rather my own stupidity had led to a specific set of circumstances that together sounded like stuck weights, and he said it’s quite possible.
Well I don't think its likely. Weights will stick from corrosion or rough surfaces but the springs, in particular the heavy secondary spring has a lot of force to return the weights.

At the time I didn’t realize there was an idle adjustment. I just messed with fuel mixture until idle speed came up a bit, but it never was perfect. Probably because the timing was screwed up to begin with, like I said earlier. I don’t have an excuse…
Timing and fuel mixtures go hand in hand. A factory emisions era carb runs fairly lean idle mixtures - often as lean as 14:1 AFR. This is one reason for the higher idle rpm -it makes up for the lost power.
If you can run it a bit richer, it should be strong and smooth at relatively low rpm. With an automatic, when placed in gear the engine should not lose much rpm if any.

When you adjust fuel mixture, adjust out (CCW) until its smooth, then adjust in (CW) until it drops rpm or vacuum. Adjusting in toward lean the drop off will be more sudden and noticible. Then turn back richer 1/8 to 1/4 turn from when it began to drop off. In gear (under load) it will need richer.

If the idle mix screw is not very responsive when screwed inward, then throttle is probably too far open.