I got the most amazing male from hell star, big and super vigorous, super stinky too. I crossed orange chem with it and I've got a seedling of the mix going now. Hoping for a female hell star next.
They'll usually pin in gaps like that due to the microclimate created in said gap, its up to your personal preference to leave them there or not but it won't hurt either way. You can also fruit right in the bag I used to use a tek called "major myc's methods of madness" using bags with great...
You can start directly under hid lights, the problem comes from going from cfl or other less intense light to hid. Seeds start in full sun all the time most folks choose t5 or cfl for cost reasons but if you start them from day 1 under hid they'll never skip a beat
You can reuse all those, I don't even wash the stuff anymore until maybe after harvest but if you run a sterile res you'll want to clean it if you reuse it