As far as i know, the USK checks the whole context&background of the violence.
If the game is really just like "uh,here are some zombies, here are some weapons, go shoot 'em" then gore will be a problem.But if the splatter is only circumstantial or you have a good reason fpr shooting their limbs off, then it's most likely going to be OK.
And i can remember the Jedi Knight games having a rating of 16+, and there was no blood, but you could use your lightsaber to chop their arms/legs/heads off.
About this slowmotion feature: if you are still in control and it's part of the gameplay, then it's a problem.But if it's only a cinematic effect where you can only sit back and watch it anyway, you could simply put a blur filter/fade to black over it.
And by the way,the usk testers play the game, fill in a formula and prepare a presentation for a special and random commitee, and this committe then discusses about the rating, and if they want,they even play a few rounds of the game.But i've heard multiple times that the testers also gives advice to the developers after they tested their game about what should be done for which rating, however, since the committe decides, the newly censored version could still get the old rating (or no rating) again.
Or you could simply put an cel-shading filter over it and go the team fortress way:
[url]http://schnittberichte.com/schnittbericht.php?ID=4539[/URL]
oh, and one of the german political parties wants that addiction is going to be considered in the rating (they also wants an 18+ rating for world of warcraft, by the way)
I hope my english was understandable.