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This can be a soil pH issue, but it resembles overwatering damage, more. Saturated soils can prevent plants from completing important cycles. Treating with fungicides would be ineffective, so you may want to solarize the soil in between crops.

Here are some articles that will help:

https://www.gardeningknowhow.com/garden-how-to/projects/how-to-solarize-garden-beds-to-eliminate-garden-pests-in-the-soil.htm

They likely aren't getting enough light. You may want to invest in fluorescent tubes or LED grow lights to hang a few inches above the seedlings.

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/leggy-seedling.htm

Yes, go ahead and cover it before the sun really does it in! However, if it completely froze, there isn't much help. If the roots didn't freeze, you can cut off the frozen lettuce and new leaves should grow. Lettuce usually will tolerate a light frost, so if it wasn't prolonged, your lettuce should be fine. Just trim off any damage.

https://www.gardeningknowhow.com/edible/vegetables/lettuce/lettuce-and-frost-protection.htm

This article should help:

https://www.gardeningknowhow.com/edible/herbs/garlic/controlling-pests-with-garlic.htm

Generally, The staff will go and edit those types of things as it is encountered. I will do so now.

This article should help:

https://www.gardeningknowhow.com/edible/vegetables/lettuce/bitter-lettuce.htm