Neville C, as far as I know and was always taught, the era of acharonim ends somewhere around 1800. I’ve never heard 19th century authors referred to as acharonim.
I did not write or imply that nobody after Shach & Taz is studied in Lubav. I don’t know how you jumped to that conclusion. But Lubav tradition is that <i>all</i> the generally accepted seforim until Shach & Taz were written with ruach hakodesh; after that there started appearing seforim that were written without it and yet hashgocha protis did not prevent them from becoming popular.
I wrote that the earliest sefer “not held of” in Lubav is Chayei Odom; surely the obvious implication is that there are no earlier seforim which have that status. I don’t know whether the Mogen Avrohom or the Pri Megodim were written with ruach hakodesh, but I do know that both are standard fare in Lubavitch.
NP, you ask how one knows if one has picked the wrong rebbe. Let me ask you how you know when you have boarded the wrong plane, or taken the wrong road. At first you don’t know. It may take some time before you notice that you don’t seem to be where you thought you should be, and you start to wonder a little, to look around you, to try to pick out landmarks, and figure out what is wrong. Eventually you realize what is wrong and you get off at the next opportunity and figure out how to get from where you are to where you thought you were going. But one sign is that, at least according to reports, and contra Neville, they <i>didn’t</i> learn Rashi, or even chumash. That ought to have tipped people off.