Weed control in a newly seeded area typically means pulling the buggers out. Don't let the weeds go to seed or they'll just compound the problem.

I doubt that the top dressing caused the germination problem. There may be an issue with the soil in that area. You might consider getting a soil test for that area to see if something is going on.

Here is a link that might be useful: Soil testing

Weeds are an inevitable part of every reno. A blanket spray of Tenacity at the time of seeding would have gone a long way in keeping the weed pressure down, but that's water under the bridge. Just let them be for now, they won't go to seed anytime soon. At your first mowing you will remove a lot of the weeds' tops, and soon after you can apply certain herbicides if you want, but the frequent mowing should effectively prevent seed production. Most of them will die at the first frost.

The soil test in that area revealed nothing out of balance.

I tend to think it was the heat that stalled germination. I watered that area frequently, even 5-6 times when I was home on an 89-degree day...but I guess the heat may still have gotten to the seed even with all that moisture. I have never seeded this early before (last week of August), so there were a couple of unexpected challanges.

Mowing isn't even going to touch the spurge (if that's what it turns out to be), tiemco...

True, the low creeping weeds will either needed hand plucking or herbicide at some point, but I would still wait till after the first few mowings.