Flamme might still be Voll's wife if we assume that a 600 year time skip had happened just before the panel where Flamme visibly ages and dies.
We know that Flamme was rumored to have been dabbling with immortality magic. Maybe she found a way to extend her life by 600 years at the cost of permanently losing all her mana and she retired with Voll?
Your theory may be right, and I have to agree: Flamme might really have managed to extend her life to live a few more centuries.
However, remember the time she had Frieren under her wing? I recall that was about a thousand years ago. And Voll is confirmed to be 400 years old in the present time. If she extended her life by 600 years, she'd die at a time where the brave dwarf is a newborn. If you want to give credit to your theory, Flamme would need to live another 100-150 years to be his wife.
I, on the other hand, sincerely hope that Frierien never falls in love or realizes that she felt anything besides affection for Himmel or anyone else.
Just imagine the pain of loving someone who has died - or will die soon, from their perspective - and not being able to do anything but live in their memory forever. Each day would be a slow agony, and for her it would not just last a few years or decades... but millennia, with the perfect memory of an elf.
The chapter said that Voll had started to guard the village at the request of his wife 400 years ago. It didn't say that he was born 400 years ago. He was already a married (or recently widowed) adult by that point.
He's probably at least 475-550+ years old now. Given that the average dwarf's life expectancy is 300 years he's one old dude for a dwarf.
Hmm, now I wonder if maybe Flamme had given Voll the same longevity treatment that she gave to herself..?
Seeing Frieren seemingly happy chatting away in different places with Grandfather Voll was quite enjoyable. A tad bittersweet, but still, she is rarely so expressive.