First, I would like to make some clarifications, since it sounds like there's some confusion about what's what...
- A successful riposte makes just a normal attack and is not a guaranteed hit. (reference & reference)
- Slippery Attacks only affects offhand attacks that are riposted. If you train 5 ranks, they will be missed 100% of the time, but mainhand attacks will still be riposted like normal. (reference)
- You can Strikethrough a riposted hit to still do damage, but it won't prevent you from being hit, unless Slippery Attacks triggers during that attack round (nothing special there). (reference)
- If you are behind an enraged mob, you can attack them just like normal, and they will take damage just like normal. (reference)
- There's virtually no chance to avoid an attack from behind, either via riposte or block/parry/dodge. (reference)
This is all how it is currently in the source, not necessarily how it should be.
As far as not being hit during riposte, I think the biggest thing we need to look at is what's your chance to avoid a hit normally? For example, if you strip naked during enrage, does it make any difference? Also, what it you strip all AAs, especially avoidance/mitigation?
One other thing that might help is to turn on combat logging, and possibly adding some additional logging in the source, to see what exactly is happening with the hits (what's the damage before & after being defended, etc).