Sounds smart to me. Seeds are seeds.

That was my concern that the seed may not be as good a quality.

I would grow the plant and save the seeds. You can always compare them with the seeds you already have, but these will obviously be fresher and you'll have lots to see you through.....

Bolting is the desperate attempt by a plant to produce seeds before it dies. If that produces inferior seeds, it was not a smart decision by the plant. It turns out that for lettuce seed production, higher temperatures are usually considered necessary, because that's how you get it to bolt, which is how you get seeds.

https://www.slideshare.net/S7w4X/zwv252

"While the vegetable crop is thought of as a cool season crop, lettuce requires warmer temperatures than many of the other cool season, dry seeded crops, to fully realize it's potential as a seed crop."

Plants that bolt early are skimpy because the plant is plowing it's energy into making seeds, to ensure survival. That skimpiness is not a genetic quality of the plant.

That being said, I believe that very high temperatures can actually damage seeds.

Yes, high temps inhibit germination. But low temps inhibit seed ripening. I've also found ripening takes ages after the flowers have died. At least it does in my climate.

My old plants are getting taller everyday and starting to form bud-heads. I was going to let them bloom just for fun and to feed the tiny wasps that frequent tiny flowers, but now I just may wait for the seeds before pulling them out. An experiment is in order!

Question... is the length of day (light) a trigger for bolting? Summer solstice? I've tried growing lettuce in late summer, and it never grows with the same enthusiasm as Spring lettuce, even with mild temps.

How hot will it get? Will you get 100 F degree temps? Keep them watered, they should grow okay unless you mean extreme temps. Lettuce grows faster in heat, they don't just die. Trying to keep them from bolting so you can eat the succulent pretty leaves is what humans desire, not the plant. Lettuce flowers because it doesn't want to dry up and die in Summer heat before it can produce seeds.

German scientists back in the 1930s established that day length had a primary role in bolting, but high temperatures do have an effect. So you can force bolting with high temps. The issue is cumulative hours of sunlight. If you take lettuce, in the hot summer, and cover it opaquely for half the time, it will probably not bolt. I would assume that lettuce raised in more shade would also be less likely to bolt. With regard to heat, bear in mind that soil temperatures in the fall can still be quite high.