As far as what spf of sunscreen to use on your indoor starts, I used dappled shade to start, and keep an eye on them. I will give them a couple hours a day and gradually increase--I think even though it's too early to put them out and fully harden them off is still okay because they will be that much more ready for the beating sun and easier to harden off later. In fact, mine are outside right now, stretching their little cotlydons and reaching for the sun--and their second leaves are really growing out now. They may get a little sunburn, but as long as it's only a couple hours, it won't be too bad and they'll recover fine.

Yes, I've used them in the past, though not so much to shield fruit after plants have lost their leaves. I've merely used them when the temperatures start staying consistently hot---say 95 degrees or higher---in order to keep the plants and fruit cooler and to reduce the stress. I actually started out using old bedsheets, and then later on I used floating row cover, and then after that I moved on up to shade cloth. About 6 or 8 years ago I bought shade cloth and put it over both tomato and pepper plants and they stayed a lot happier and produced more fruit in July and August than they usually do. The shade cloth I currently use in the garden is a 40% shade cloth and the one I use on the green house is a 50%. For me, shade cloth works better than row cover because it is stronger and more sturdy and holds up better to strong wind than most row covers do. However, I was happy with row cover when I used it and I was happy with plain old bed sheets when I used them. It is just that when they got old and tattered and started getting holey, I moved up each time to a stronger type of material so that it would last longer. I think the main way that shading helps is it reduces the temperatures the plants are exposed to which, in turn, reduces their stress. If you put identical thermometers on the ground underneath the shade cloth/row cover and out in the open in full sun, you'll see how much of a difference it makes. You also can use row cover or shade cloth to save melons or winter squash if they lose too much of the coverage they get from the plant foliage. I always suspend the material a foot or two above the top of the plants I'm shading because if you don't do that, sometimes too much heat can build up beneath the cover and can cause the plants to overheat and wilt. In winter or early spring, I sometimes let row cover just float directly on top of the plants especially if I only am using it overnight for freeze or frost protection. In late spring through mid-autumn, though, I erect a framework to raise the cloth higher above the plant, allowing for better air flow. If you don't do that, on a hot, sunny, wind-free day, the plants can roast under the sheets, row covers or shade cloth. With the row cover, there always will be some heat build-up, although the most lightweight row covers don't let too much heat build up. I have some extra heavy duty floating row cover that gives 10 degrees or more or freeze protection and it is very thick and heavy. If I leave it floating directly on plants, once the temperatures are in the 80s, the plants can roast and be severely damaged in just a few hours. So, with that one, I always use hoops to hold it well above the tops of the plants. You also can use row cover to keep the ground beneath it cooler in order to get cool-season seeds to sprout in the ground in August for the fall garden. Soil in full sun here in August can be too hot for good germination of cool-season seeds. I use row cover (and shade cloth and old bed sheets) in many different ways. In our hot climate, you've got to use every trick in the book to fight the climate and give your plants a little bit of help.

I guess it depends on what you call "done for". Tomato seedlings are almost 100% dynamic mass & carry on fairly efficient photosynthesis in the stem tissues, too. Those tissues may have also been damaged. The leaves will regrow from buds in leaf axils, if you want to wait. If you're zone 6, you still have time to start them from seeds if you want, or replant if you bought them in cell-packs. I don't know what your plants look like now (how bad), but I would strip all the lower leaves off so there are only 2-3 pairs of leaves at the top of the plant, & replant them deep - so only the upper leaves are barely above the soil. The entire length of the stem will grow roots & you'll have a much stronger root system. I do this with ALL my tomatoes. Al

I had a similar sunburn problem with my 1st batch of seedlings. I had been growing them inside the house under a fluorescent light and thought that I would set them outside on a overcast cloudy day in order to harden them off. Unfortunately, as Murphy's law would have it, the weather turned bright, sunny, and hot while I was at work, and I wasn't able to get back home until 10 hours later. I took the seedlings back inside and kept them in the house under grow lights afterwards, but over the next few days, all of the leaves turned pure yellow. Fortunately, the later new leaves that grew out were nice & green, but the old (yellowed) leaves gradually wilted, and I eventually pruned them off. If you can, keep them inside under some grow lights until some new green leaves appear. The plants might be set back in their progress for a few weeks from where they were previously, but they should recover. I pruned off my older yellow leaves after a couple of green ones had popped up--on the theory that the yellow leaves weren't going to be able to do any good photosynthesis for the plant and were just wasting the plant's food, nutrients, and energy in order to keep maintaining them. Mine recovered beautifully about 2-3 weeks later and are now sticking up 8 inches but are quite stocky and wide, with lush green leaves (they are a total 6 weeks old since sprouting). Also, as you probably know at this point from your own experience, on your next hardening off attempt, but sure to gradually expose them to only a few hours of overcast light and phase them into full sunlight (preferrably over a 1 week period, but if you really are in a rush, then 3-4 days at the fastest).