Wow, all this information and after reading the whole thread I'm not convinced that you even have spider mites. Is there a way you could post a picture of your plants and/or damaged leaves? If you have a spider mite infestation, there will be tiny white specks on the top of the leaves where the mites have sucked all the juices out of the leaf from the bottom. On the underside of the leaves, you will see tiny black dots, that look like slow-moving pepper.
If the two spiders you saw were not actually on the plants, and they were bigger than a small speck of pepper, then they were NOT spider mites.
I have had regular spiders spin webs on my plants, they are a little annoying but they are not harmful and don't require poisons. There's no sense putting poisons on your plant until you have made a positive ID of spider mites or another harmful bug.