Definitely agree with the fact that missing your shade open skills is a bad play, but I disagree with the idea that you need 3 shades to have a good lawtia rush. I feel that notion is very outdated, as there are tons of good lawtia lvl 2 tech cards now. You can definitely run a good lawtia rush with just 2 shades, 3 shades are not a necessity. I've actually run several good lawtia rushes with just 2 shades, and some games I've actually only played one due to it being more advantageous to play something else. Undeads can especially get away with it as they can use broken priest instead.
I've been playing undeads quite a bit recently in folrart, and as of this writing my current undead file is 23-4. It only uses 2 shades, does not use salamander (I find it to be a waste of an sp to increase gowen when lawtia has plenty of good tech cards), does not use death's embrace (not a fan of that card) and it uses skeleton archer. I'd also reject the notion that undeads are not viable somehow. I've faced a lot of this file type's weaknesses and still managed to win, including max hp hate cards like verlaat, talisman, Sergis SS, Francis, and attack immune cards like Mage soldiers and EX Mermaid. I've even beaten Gaiessoul abuse files with undead. I'd go as far as to say that lvl 3 undeads are not just viable; if played well, they're more than capable of being meta.
Anyway, I agree that Hellsmoke, shade (you only need 2 imo, 3 is a personal preference), EX CAK, Priestess of Madness (although you can use 2, 3 is not a necessity), broken priest, and Lich are all definitely necessary to make the best lvl 3 undead file possible. If you're lacking these cards, the file is not going to be meta. After that, it's a matter of personal preference. The other cards added as "must-haves" are certainly good options, but there are other cards that are just as good in this type of file.
As for SS lineup, I'd say you want a lineup that takes advantage of undead's strong field presence (due to not needing to revive), while taking advantage of their strong return from cemetery abilities (so a little extra lp in the end compared to other rushes).