I think slugs eat just about anything. :-/

Looks like botrytis blight (called gray mold) to me. If so, it is typically caused by poor air circulation, overly moist conditions, etc. Locations which receive a lot of rainfall and reduced sunlight might be affected more than others.

I would detroy the plants. Spider mites are hard to kill. Difficult but not impossible to get rid of. A really thorough spraying with a Neem oil spray will get nearly all of the mite population. [Look in the Organic section of the garden products for sprays that claim to be effective against mites, spider mites, or tomato russet mites. You'll probably have to check the ingredients for Neem oil.] First, remove and destroy all the leaves that are beyond saving. Spray under each remaining leaf -- that's where most of the mites will be found (spray the tops also). Spray in the evening or on a very cloudy day, as bright sun on the leaves (when the spray is still wet) will burn them. Spray again after the interval recommended in the instructions. I use Garden Safe's Fungicide 3; I buy the concentrate and mix my own. Some weeks back, I was surprised to see the concentrate at Walmart.

I have always planted marigolds around my tomatoes... don't know if it has actually helped them, but it certainly doesn't seem to have hurt them at all. My tomatoes have been trouble-free except for late blight during last year's wet summer. I love companion planting; it makes for a beautiful garden and attracts lots of pollinators and beneficial insects, if nothing else. Sometimes my N of 1 finds good results from something I try... like the nasturtiums acting as a very effective trap crop for aphids, or the squash interplanted with flowering radishes which were untouched by SVB while the others were all hit. Coincidence? Maybe, but it certainly didn't hurt anything, and might have helped!