Where are you situated? What is the temperature there?

I can well imagine too-strong fertilizer burning and killing your young transplants, but if they grew well until now, I'd look for some other cause, like heat or dryness in the soil.

I agree with Itilton. Your fertilizer is fine and is probably washed away by now.
The lower leaves will get that way as the plant matures. I pull them off. Make sure they are watered and you may want to put a shade cloth over them if you're in a hot area. Most lettuces have gone to seed (bolted ) by now and people are starting on their new crops for the fall.

Also, check under the lower leaves for insects or other critters that may be eating the roots and stems. That can cause wilting, too.

I'm just wondering if you might prefer to try gardening organically. Good composted cow manure, compost, etc, will add humus to your soil and provide nutrients that are gentle on your plants. This page could help:

Biodynamic French Intensive Method

Here is a link that might be useful: Alan Chadwick