I'm not sure that the tall Nicotiana is a hybrid. I know crimson king is an open pollinated variety. The dwarf ones are hybrids.

It may or may come true from seed, it's hard to say.

I grow the dwarf hybrid ones and I know if I plant the seed I will not get as vigorous a plant and maybe not the color either. I don't remember exactly on the color because it's years since I've saved seeds except for a few perennials.

I never took stem cuttings. What are you going to do with them? grow them all winter and take more in the spring? or is your climate warm enough to grow them outside all winter?

robinmi,
I have never seen or read about a race of N. mutabilis that was not "mutable".
Mutabilis is my favorite Nicotiana. A stable white form would be superb!
I have grown another species from stem cuttings, specifically N. alata hybrids, successfully. I certainly hope your efforts pay off.
Alana