It's possible to argue that it is appropriate to use SIA in some of those examples, but SSA for the duration of the human race.

"my daughter just had her first day of school, and if we assume that she's equally likely to find herself in any position n of her total amount of lifetime days in school N, then it follows that there's a 95% chance that she will spend a maximum of 20 days of her life going to school"

The doomsday argument doesn't say that, even if you do use SSA with the reference class of days that your daughter is in school. You're confusing the likelihood of the evidence given the hypothesis with the posterior probability of the hypothesis given the evidence.