I mean, d6 is a perfectly legitimate move actually. It allows us to play d4, but the idea is to play Nf6 and d6 controls the e5 square. Many grandmasters have played like this and it’s not that of a bad option
I've seen that d6 is actually playable against the Alapin but it gives white much more of an edge than Nf6 or d5. Nc6 can be played as long as black follows it up with d5, transposing into a Barmen Defence line.
Yeah that’s true 👍
e6 is also playable against the Alapin, but impractical. As far as I know, the only good way to play it is to follow it with d5, transposing into an Advance French. Grandmasters (who tend to know how to play all openings) might do this occasionally as a move order trick for a particular game against a particular opponent, but it doesn't make much sense for the rest of us to learn a whole new opening just to deal with a particular sideline for which there are two other, simpler responses that are at least as good.