Distressed cat and first time owner

Hello everyone, I'm brand new to the forum and as a cat owner. I adopted a cat from a local shelter about five days ago. He was found with severe fleas and mites, and I have to give him eardrops still due to an infection caused by them. He is 8 months old, and a very sweet boy. Today I found black specks around his chin and a line from around his eyes to his ear. I did the water drop test and found them to be flea dirt. However, I had to rub with a warm cloth and later use a flea comb in order to get them loose. Now a lot of dirt came up, but I haven't seen a single sign of a flea otherwise.

I contacted the shelter again and administered medication, and am in the midst of trying to keep him isolated so I can clean and get some answers, but the huge blizzard has effectively shut vets and other shops down for a while. So, the few questions I hope you all can help me with is:

1. The medication I gave to him is supposed to kill fleas in 6 hours, but I got halfway through vacuuming and spraying the furniture with flea carpet cleaner before the vac broke, so do I continue to keep him isolated through the night?

2. Is there any chance that this is left over from the shelter, and I can just keep combing the flea dirt away and be done with it, or is this a whole new round of fleas?

3. If that's the case, would it have been from the shelter? Because we've been snowed in for a couple days now, we have no current pets, and he's only met one other person besides myself and they don't have pets

4. Is there anything else you recommend? He's very distressed, poor thing, and I'd feel horrible keeping him locked up all night, but I'm waiting for a replacement vacuum in the morning, and am basically in limbo until then.

I know this is a lot at once, so thank you for your time and answers!  

What flea medication did you use? I'd recommend giving Capstar and a decent spot-on product like Advantage or Frontline (of course, if you've given a spot-on already, don't give it again until next month!). Capstar is wonderful . Amazon has the best price, I think.

Yes, it's possible the flea dirt is leftover from the shelter. I would recommend a bath to get rid of all the shelter nasties, but if he's already scared you don't want to really freak him out, and if you just gave him a spot-on you have to wait a few days before bathing. So just comb him every day with the flea comb, that should get rid of some flea dirt and you can check for live fleas.

They gave him Capstar at the shelter according to his record, and it cleared him up in a week. The shelter gave me Vectra for Cats, but they only left me the tube on the doorstep and e-mailed me vague directions, so I feel like I'm in the dark! They are a reputable shelter, but this blizzard has thrown everyone into a tizzy. Thank you  

Capstar is only effective for 24 hours, so it's really meant to be given whenever you see live fleas. I have never used Vectra but I think it's known to be fairly effective. So hopefully this takes care of it for now! Of course you have to keep up a good flea prevention plan all the time.

Things are starting to make sense now! Hopefully now we're back on track