My sweet basil is growing rather profusely. How do I know when to repot into a bigger container?
You can lift the root ball carefully from the pot. If the roots are maintaining themselves in the shape of the pot, it's time to repot. Increase only one pot size at a time.
One of my basil plants in a pot has tiny black spots on some of the leaves. They do rub off. But the new baby leaves are being eaten, I think, by earwigs. How do I get rid of earwigs and what are the black spots?
The black spots could be scale insects, or aphids, or even spots of sooty mold. A good treatment of neem oil should take care of whatever those are, as well as unwelcome diners.
https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm
I am growing basil in a pot outdoors and it's getting pretty big. How do I know when it should be transplanted to a larger pot?
There are really no hard and fast rules for repotting. For most plants, if you can remove the pot from around the roots, and the roots are all matted inside and retaining the shape of the pot, that is called root bound, and repotting is probably a good idea, for most plants. Other clues can be there's so much plant that the whole thing keeps falling over, or it doesn't seem to stay hydrated no matter how much water you pour in. Other than these things, most gardeners recommend that plants be repotted annually, to renovate the roots and replenish the soil. Here's an article that talks more about repotting: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm
I'm preparing to cut my basil plant for drying and need to know if it should be washed before drying it.
Yes, you should wash herbs before drying. This article should help your project: https://www.gardeningknowhow.com/edible/herbs/hgen/how-to-dry-herbs-various-methods.htm
There are no raised bumps underneath! Leaves were fIne until the end of summer!
Unless you know for sure what the infectious agent is, I wouldn't eat the leaves. And of course, certainly don't eat if you've sprayed with a chemicals. This article on basil diseases may be useful to you: https://www.gardeningknowhow.com/edible/herbs/basil/basil-diseases.htm
Is it true that you should cut back your basil plant (grown inside) so that it will not get bitter?
You should do that with any basil plant. Cutting it back regularly will keep it from bolting (flowering). Once it bolts, it starts to become bitter.
Something is eating my basil and I'm not sure what it is. I put copper around my basil to prevent slugs but but still it's getting eaten. Can you please help me?
There is still a possibility that the slugs are still getting to it. You may want to set out a small pan of beer inside the copper barrier to test whether it is slugs or not.
It could be rodents as well. What does the damage look like?