So I think I read to deep into somethings so here it goes.

Combat damage. I sacrificed my Skavenslaves in order to prevent all combat damage assigned to attacking units. By killing off some of his units there was a point where redirected damage was coming back my way. Is this also covered my Skavenslaves ability? The combat damage also came up in the same game different scenario where I was trying to kill the Sword Masters of Hoeth which have a card description along the lines of cancel all combat damage assigned to this card. I was trying to use Lokhir Felhearts ability to assign -1 hp per development in order to kill him. In the end I just canceled the Swordmasters card effect and killed him just to make sure it was fair.

Redirected damage retains its type. So if his defenders redirected combat damage to your attackers, then yes, that would be cancelled due to your Skavenslaves' effect.

HP reduction is not damage. HP reduction does not put damage tokens on a unit, it effectively lowers the printed HP number on the card. Damage cancellation (like Toughness) and redirection do not work against HP reduction. The Sword Master's effect cancels combat damage, but does nothing at all against HP reduction. And even if HP reduction were damage, or if Lokhir's effect dealt damage instead of reducing HP, this effect happens at the beginning of your turn. Only damage that is dealt during the damage step of the battlefield phase, and counterstrike damage, are considered combat damage.