Well, the Night of Rage was in 2039. We know that Frosty was in her 20's in Harlequin which took place in the early 50's. So it's plausible. The only questionable part is if Frosty's mom is immortal or not. The idea of any powerful immortal initiate being able to be actually threatened by even a largish group of Humanis seems unlikely.
The entire circumstances seem really weird when you think about the death of Frosty's mom.
We know that Frosty had a magical McGuffin in her so Ehran could keep tabs on her. But it can be assumed she was born in the CAS, while Ehran was in Tir Tairngire helping found it. So when did Ehran have time to enchant Frosty's bones? Also, why didn't Frosty's mom stay with Ehran?
Lets pretend that Frosty's mom is an immortal, what can kill an IE? Dragons, other IEs, and Horrors.
Horrors is probably less likely, though possible, since the Horrors metaplot didn't become a real threat until the 50's.
Dragons, is possible since Sirrurg started to hunt IEs in the 40's and did that for basically 30 years. But Sirrurg is usually pretty obvious about it, though Dragons of the Sixth World did say he was more thorough about not leaving any survivors to hide his involvement in his attacks after EuroAir Flight 329. But the problem is that the timeframe is off since Frosty would have only been an orphan for 10ish years. Though, EuroAir Flight was a flight from London to Atlanta (aka CAS) so maybe the mage that held off Sirrurg was Frosty's Mom? I guess losing your mom at 10 or so would still count you as an orphan, so it's possible.
Other Immortal Elves. Ehran probably has a lot of Immortal enemies. Could have easily been Harlequin, or the handful of other IEs that are known, or even one that isn't known. I guess powerful free spirits can also be a threat too. Harlequin did make a pretty maleficent spirit thing that was an embodiment of his negative emotions or something, I actually can't even remember if that's a thing or not. Was that a plot point of the Sega Shadowrun game?
You know, there are a lot of plot points here that could spin into a very elaborate campaign.