For an opening to be a drawing weapon, you'll need to be able to use it. White is gonna play something like a4 or h3 in 90% of games, so the Marshall Attack won't even occur.
Now, a true drawing weapon is the Berlin wall, or the Petroff. It can't be avoided, unless White stays away from 1.e4. If he tries 1.d4, then the Slav comes into play. So Black will never have a shortage of drawish openings to pick from.
But anyway, saying that drawish openings don't exist, is just plain dumb. And converting the question into "no openings lead to draws" is even dumber, by arguing that there can be no ideal cases, and therefore no cases at all. So just stop it with the "you can't compel a draw" argument. It's only cute the first time.
Exactly! There are openings that have higher drawing chances compared to other openings. The Berlin Defense (Berlin Wall) is a prominent example.
Danish Gambit. Surefire draw every time.