That looks like crabgrass from this distance. Weed-b-Gon does not work against grassy weeds. You would need a dedicated crabgrass killer.

My landscaper said it was Pennsylvania Smartweed and suggested I use Msm Turf to address it. I misread the mixing instructions and ended up using too much which destroyed the area I sprayed. Now the question is whether to address it in the fall by overseeding or wait until spring, any suggestions?

Absolutely address it in the fall. In fact, fall is about a week away for you, so take this time to plan your approach. Spring is a terrible time in most places of the country to seed grass.

I thought perhaps nothing would grow for a while in the area where I killed everything with msm turf.

Appears you have Annual Poa. Very aggressive and produces heavy thatch that crowds out good grass types. You can applie an pre-emergent to prevent the seed from the seed heads from germinating. Best I found was to De-thatch your lawn and use a crabgrass killer and pre-emergent to remove it. Next opportunity you have to re-seed, Fall or Spring (make sure you wait for the pre-emergent and crabgrass killer has dissipated ~ 3 to 6 months) then re-seed.

So I seeded about two - three weeks ago and the grass is sprouting quite nicely. Is it ok to go over and reseed some of the thin spots?

Yes. This is the primary reason for seeding early in the fall (or late summer). It gives you a chance to evaluate the coverage and density and correct things before the fall frost later on.

Most of the seeding I did early on has grown very nicely, seed recently put down - about two weeks ago - has not shown any growth, is that seed completely gone to waste or will it show results in the spring?