To the genetics: I only remember that the genetics of immortality were discussed in
Tir Tairngire, the so-called "stopwatch complex".
Because the immortality gene is recessive, it means that Frosty's mom must have been an Immortal Elf too.
Where is it stated that there is an "immortality gene" and it's recessive?
Anyway if it is recessive, then her mother does _not_ have to be an immortal elf. If an allele of a gene is recessive it will not (or mostly not) be expressed unless the allele is present on both chromosomes, otherwise its expression is suppressed (it is dominated) by the other allele. So there are individuals that can pass the recessive allele on, though its effects are not visible in their phenotype.
What I'm getting at is this:
Is Frosty considered an Immortal Elf, just because the other IE say she's one (being Ehran's daughter), or is there some objective proof that you could check if someone is?
Since we still don't know the lifespan of an Elf, is it possible that they're all Immortal (aka don't die of old age), but those we call IE are the only ones that survived the 5th age, while others didn't (for whatever reason)?
I think Frosty gets counted because Ehran singled her out in his monitoring of what was probably offspring of immortals (maybe only his own). All the monitoring had been terminated at some point between 14 months and 40 years ago (from 2050), after the individual under observation had shown a serious illness or had reached an age of around 40. Only Frosty's information had been deleted to obscure it.
In Earthdawn there was a magical way of identifying immortals (dragonkin) and there may be a scientific method that was employed in this montoring too.
400 years as the maximum in Earthdawn, the scientific estimate in Shadowrun is several hundred years. So, indeed we can be only sure about those elves that are known to have been born before that. I don't think we even know that for Ehran.