If it is possible to get a fall crop, when should I plant? Should I add nitrogen or other fertilizer? Thank you.
Broccoli rape is not a cool season crop. It isn't related to broccoli despite it's name. You can sow seed any time directly into the garden. Here is more growing information. https://www.gardeningknowhow.com/edible/vegetables/broccoli-rabe/broccoli-rabe.htm
Planted in late August for a fall crop and got beautiful growth, but no flowers or florets.....did I just get bad seeds?
Typically, you will want to harvest well before this happens. Once it flowers, it is ruined. After this point it should be considered inedible and thrown out.
This article will explain: https://www.gardeningknowhow.com/edible/vegetables/broccoli-rabe/broccoli-rabe.htm
It started on the Brocolli Rabe, jumped to the Fava Beans, and now seems to have moved to the turnips. Im not sure if it's related, but my sugar snap pea leave have all turned yellow. What can I do to save my growth? Do I need to take out all of my plants and start over?
Ultimately, it will come down to just a handful of remedies.
Once this takes hold, it is difficult to rid from your garden. You will need to treat with a fungicide, and keep all infected, dead, or otherwise tattered plant material completely away from the area. Rotating your crops each season can help lessen the effects, as well.
Here is an article that will help: https://www.gardeningknowhow.com/plant-problems/disease/alternaria-leaf-spot.htm
I’d like to know the best NPK for my broccoli rabe plants
All the articles I saw said to plant rabe in fertile soil with lots of organic matter. Compost, worm castings, fish fertilizer, manure, and chicken litter are ways to increase soil fertility organically. These articles should help.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-organic-material.htm
https://plants.ces.ncsu.edu/plants/brassica-rapa-ruvo-group/
https://edis.ifas.ufl.edu/publication/MV033
http://www.gardening.cornell.edu/homegardening/scenec40b.html
https://www.gardeningknowhow.com/edible/vegetables/broccoli-rabe/broccoli-rabe.htm