Safers can be bought pretty much anywhere, even walmart I think. I would not use Sevin this close to harvest! It is not labelled for late use on consumable crops.
There's a mite identification misnomer amongst us potheads, lol! You are correct spider mites are red; those don't harm plants. The mites we see in our gardens are called spider mites even though they are a different species because they weave webs.
Black bugs- I can't think offhand what they might be besides fungus gnats, but those are more a greenhouse problem from what I can think of. They like standing water and soggy soil. In and of themselves, they are not harmful to plants but CAN spread disease.
Picking leaves- I wouldn't do it if it's a frost kill issue.
Back garden plants- do they get less light and less breeze? Another reason they could be suffering the cold and damp earlier on.

Also at walmart they sell Serenade which is TOTALLY safe and OMRI organic certified to use as a rot preventative. Instead of picking leaves, I'd simply treat them to prevent them rotting.

Remember outdoor plants don't always look pretty like indoor ones! But they still make BUD.