I've had mixed luck with DWC peas and would love to see how your technique is working out if you wouldn't mind a charitable picture.

The biggest worry I can see is what exact variety of lettuce you have - whether it basically is a short-day or long-day flowerer. According to this paper, it looks fine for most varieties below 14 hours light, but by 20 hours there can be problems. Higher temperature independently can further enhance bolting. So why not just try it at the 12 hours and if you're worried and can lower the temp a little, both the peas and lettuce will probably do better with that.

photoperiod of lettuce trials

I'm sure there are many other sources to know the characteristics of your varieties, even maybe try the seed company?