Window #1 with channel balance and rotted plastic parts

Thanks for posting, Safiyyah. I believe we can help. If the channel balances shot past the sash, it is because those nylon pieces broke, exactly as you deduced. They are called Top Sash Guides. I'd like to see an undamaged one, preferably removed from the window and with pictures from the front and side. That will help narrow down your options. See the video below which shows how to remove and replace Top Sash Guides. 

Now, if the balances are undamaged, replacing the sash guides should fix these windows. If any of the balances have damaged springs or end fittings however, we can replace those for you as well. These channel balances look like either our S351-27 or S390-27. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. See the diagram below for reference. In either case, for a stamp option choose 2620. Let me know if you have further questions. 

Comparison of the S351 and S390 balance shoes.

Thank you! This top sash guide appears to be a match for our 18-032. The diagram below shows it in a black finish, but it's available in white, too.

I'm afraid I don't know what the L shaped piece is. It appears to be part of the weatherstripping in some way, but it is unfortunately not something we can help with.

Front view of 18-032

Side view of 18-032

That's a good question. In truth, there are some windows that have fixed top sashes that are not made to be removed in any way. If you can't find a way to remove it, and if it won't even budge to begin with, then that may very well be the case here.