Maxperson
Morkus from Orkus
They don't have to be aware of something on another plane before initiative is rolled. You can say something like the following.I see. That doesn't work for me as an interpretation because it implies that Alert can do something beyond the words in the feat. That it will automatically make its possessor aware of creatures - even those with abilities that should prevent that character being aware of them, such as etherealness. Well, no kittens will die if we each go with our preferences
That's an understandable reading, but not really a logical claim: it ignores the logical system that the rules are laid out with. A general rule for surprise is the first line you quote. Feats create exceptions to general rules. The Alert feat creates the exception summarised in the second line you quoted. Ergo with the feat, you can't be surprised - regardless of what you notice - while you are conscious. Otherwise, logically, if you somehow didn't notice something - while conscious - it must surprise you. But the feat says that can't happen!
The version of Alert that I think would meet your purpose would read something like "While you are conscious, you always notice threats at the start of an encounter".
DM: As you walk through the brush you see a fresh bone half hidden by the grass. The hair on the back of your neck begins to rise. Suddenly, 3 creatures fade into view around you and your companions. Roll initiative.
You've alerted the PC to the danger just prior to the ceeaturea fading in. Those without the feat are surprised. The one with it isn't.