It depends on what type of lettuce it is -- part of the issue is that if the lettuce is touching plastic, it will rot quicker, so I wrap it in paper towels, then bag it (but not sealed), and keep it in my crisper.
For whole heads of lettuce (iceburg, butter, red leaf, etc), I just wrap the whole thing in dry paper towels, then shove it back into the bag from the grocery store or farmer's market. I then pull off leaves as I need it, and re-wrap it. It stores for well over a week this way.
For mescalin mixes, arugula, or other individual leaves, I'll wash them, dry them, then unroll enough paper towels to spread the leaves on, then roll up the whole thing, and bag the roll (again, not sealed), and keep it in my crisper. I can probably get a week out of it this way.
(all times assume you're not buying from a store where it's been sitting on the shelf too long before you buy it; I get my lettuce when I can from the local farmer's market)
So, to answer the specific questions:
- keep the bag open; you don't want moisture to condense inside the bag, as it'll make the lettuce rot faster.
- I get better storage time with heads of letuce keeping them whole. If you're going to be eating it all within 2-3 days, it probably doesn't matter, and for loose lettuce, I find it more convenient to wash it as I re-pack it anyway.
- You never want to store lettuce wet ... you might be able to store it completely submerged, but damp will lead to it rotting faster.
Wilting in greens is triggered by temperature, pH, and salt content. To reduce wilting, you can cool the vegetables or shrimp, make the vegetables more acidic*, or decrease their salt content.
My suggestion would be to cool the shrimp with an ice water bath or cold running water. This is the most traditional approach for shrimp salad. Alternately, you could apply an acidic dressing before topping, but wait to salt it until the shrimp have cooled to 140F.
Alternately, wilting of vegetables adds a rather interesting variation in flavor or texture to a salad. This technique has been desirable and in vogue at various times.
Why does this all happen?
Plant cells wilt when cooking breaks apart the cellulose-based cell walls, and allows water to escape, causing them to soften. To quote On Food and Cooking (pp 282):
When the tissue reaches 140F/60C, the cell membranes are damaged, the
cells lose water and deflate, and the tissue as a whole goes from firm
and crisp to limp and flabby.
Acids will impede wilting, because the cell walls are held together with hemicellulose, and it is "not very soluble in an acid environment" (pp 282). Table salt is a problem, because "its sodium ions displace the calcium ions that cross-link and anchor the cement molecules in the fruits and vegetable walls, thus breaking the cross-links and helping dissolve the hemicelluloses" (pp 283). Calcium has the opposite affect, so if you can use hard water or add calcium salts, do so.
*Acidic solutions will reduce green colors(On Food and Cooking, pp 280-281), but preserve texture (282).
Best Answer
Yes it is still good.
A lettuce that is kept outside (as in a farmer's market) will get wilted outer leaves, and the merchant will usually cut them off to make the heads nicer.
Depending on the resulting size; if they cut out too much compared to other lettuces, I might ask for a lower price if sold by the unit.
Personally, I will buy the lettuce with as much of the outer leaves as possible, even if I remove them at home, they will act as an outside protection against the elements.