The 1918 Spanish Influenza lasted over a year in several waves, each wave apparently being a different mutation of the last. There was no obvious sign it was over at the time and I think it may have been fall of 1920 or later before people realized it wasn't coming back.

There were a couple of areas that managed to avoid deaths via isolation. I can't remember the details but it was islands. I think some Eskimo communities escaped devastation by killing any outsider who persisted on approaching.

30% may be reasonable absenteeism - that's about the overall infection rate in the 1918 pandemic.

Predicting which virus is likely to appear is the entire basis of the annual vaccine production. Only vaccines for a very small number of strains is made (one to three I think), and if they guess wrong too bad.