I was researching this question about "not losing" and I made a little discovery (at least for me).
Although Kasparov is probably the best player with the white pieces (statistically) and this is not much of a surpirse, it was a bit of a shock that Fischer is probably the best player with black!

Of course, a lot of this has to do with his will to win every game but I think it mostly has to do with his choice of the Najdorf and the KID when he was at least 10 years ahead of the curve (theoretically speaking).

Perhaps the best way to not lose is, in fact, to play agressive, sharp systems as black?
Then again, Kramnik is also fantastic with black (in that he rarely loses) but his reportoire is not sharp.
Hmm...

I'm afraid that the repertoire isn't so important, but rather the will and the preparation of the player.
Although, it still seems pretty likely that the Queen's Indian would be a better pick than the King's Indian when trying not to lose.

Difficult question. As you say, solid openings can easily give passive positions which in the end can be difficult to hold. Two suggestions might be the Philidor and the Old indian, but I guess they fall into the 'too passive' category.

At top level, I seldom see Ruy Lopez Marshall lose. It's not a safe opening, but many lines seems to give drawish endgames.

Btw, which semi-slav lines were you referring to? I never looked at the semi-slav as a drawing weapon.