There's more good news, too. Many weeds dislike being mowed, so when you start mowing reasonably regularly at least some of the weeds will die out. I ground my teeth during my reseeding last fall, but by the third mow the weed percentage was way down. Most of the remainder didn't survive the winter. You can pluck out anything you can reach as long as you don't have to walk on the patch or lean on it. Right now, don't put any pressure on the new grass at all. Spring seedings are touchy enough without adding stress. Of course, everything but the first few feet can't be done, but it's better than nothing.

Did you roll the seed down? Did you water 3x per day for 3 weeks while the seed was fresh on the ground? Where do you live? What kind of seed did you use?

Funny you post this because I was just having the same thought with my newly seeded lawn. Yesterday I was admiring the new seedlings and noticed our cherry blossom is just starting to lose it's leaves (I'm in SE PA so likely a bit warmer climate than you). My guess is that while yes it would be better if no leaves were blocking the sunlight, you don't really have much choice as the alternatives are probably worse for the grass. Walking is definitely out of the question as you would probably flatten the blades with little chance of recovery, and the blower would do the same. I'd just leave it alone unless you get an inch of leaves, then you probably have to act. As for the grass growth there's a lot of reasons why this can happen. A localized spot of nutrients (for example the spot where the fertilizer pellet was dropped or a bug died) is a good culprit. Remember the seedlings at start have pretty much a single small root to pull all of the nutrients. During that time if the seedling cannot get all of the nutrients in a proper amount the growth will be stunted until it gains access (a worm/microbe cruises by, you water and some nutrients migrate by the root, etc.). Other simple things like a leaf or piece of straw can block the sunlight to a very small area which could cause the blade to grow slower than one right next to it. And as you mention soil density can hamper or help the growth of a new seedling. You probably don't have to worry about seed to soil contact as that should be pretty uniform using a slitseeder, but it's also possible that some of the tines (or whatever they call them on the slitseeder) may be slightly different and so you get a mosaic on the lawn due to being slightly deeper or shallower. Bottom line is 8 days for just about any grass is nothing to be worried about. If in another week you still see nothing, then it's time to think about maybe overseeding those areas, but for now just relax and enjoy the show!

No rolling! You would have done that at seeding if you were going to do it. Now you could crush germinating seedlings. Stay off the yard as much as possible. KBG takes 2-3 weeks or more to germinate so you should give it a chance to do so. You won't have a chance to reseed as it will be too late in the season. Looks like things are coming along okay. Ideally you should have planted in mid to late August and that would have given you a chance to fill in bare areas with a little more seed in September. Hold off on cutting a little longer if you can but if the rye is getting too long, you have to cut it. Don't water the day you mow until after mowing. Make your turns on hard surfaces if possible or make slow three point turns.