The tested sample of the 1918 flu was recovered from the Alaskan Brevig Mission, a tiny isolated Inuit settlement.
During the five-day period from November 15-20, 1918, the 1918 pandemic claimed the lives of 72 of the villages’ 80 adult inhabitants.
So my question is, was the recovered virus genome from the first generation? What's the best guess as to the properties of the second more deadly wave of a mutated virus?
No idea. Sorry.
None of the material I could find written about the isolation of the 1918 influenza virus indicates whether it was first or second wave virus. I highly suspect that it was a second wave virus, given the lethality the virus caused in subsequent mouse and chick embryo experiments after the virus was recreated.
There was quite a bit of controversy over publishing its sequence. Influenza research can be a very sensitive topic, since many researchers (and policy makers) believed that the next pandemic would be another influenza virus. The 1917-1919 H1N1 was particularly lethal (but not as much as Covid-19). We still have circulating H1N1—the 2009 pandemic was distantly related to the 1917-1919 pandemic virus—but it is much attenuated from that time.