Lettuce is more tender than spinach so it cannot handle a frost/freeze. Bring in the lettuce to avoid below freezing temps. A frost won't kill the spinach but you might as well bring inside too so that the planting medium doesn't freeze up.

I've never used coconut coir so someone else may know that answer.

Bringing the plants in and out does them no favors. Wait to take out until they can remain outside continuously. The seed package should detail the appropriate planting time, generally a factor of when your last frost date may be.

If these are intended to be grown in pots, then it is best to use a dedicated potting mix that includes ingredients to provide proper aeration and fast drainage - peat or coir, perlite and even fine bark. Although not considered very favorably by experienced container gardeners, Miracle Gro potting soil is completely adequate for this purpose.

"Lettuce is more tender than spinach so it cannot handle a frost/freeze."

Not really: possibly for certain varieties, but all the lettuces I've ever grown can easily handle frost and temperatures in the mid-to-upper 20's if properly hardened off in advance. The typical date for transplanting lettuce starts here in upstate NY is mid-April or so, which is a full month before our average last frost date.

I'm just going by my past experience. Every year I'll acclimate my seedlings to outdoor conditions and it helps not hurts. As for the lettuce my mind was on autumn planted lettuce of which has been damaged by frost in the past. Younger early spring lettuces do take a little frost on the chin better, esp when planted in the ground outside from seed and have adapted to early spring temp swings.