1) The Blood reavers get +1 Attack when a TOTEM hero is nearby, and the bloodsecrator has a buff that adds +1 Attack AND has the totem keyword. So do the reavers get 3 attacks then?
Yes and Yes
2) paired with an aspiring deathbringer, could i even buff them to 4 Attacks per model?
Let the Blood Flow!

(Yes)
4) Can I break the game by taking -say 2x 40 reavers and 3-4 bloodsecrators and let them stack their buffs? I certainly hope this is NOT possible, but i cant find anything from preventing you to do so.
It's a pretty decent combo, but there are things that'll do pretty well against the army still, not really a broken combo.
As for 3 I have entirely forgotten if the Mount's attack counts for the +1 to each weapon thing, so I will leave that unanswered.
The Khorn Bloodbound are one of the most synergistic armies in the game, and many of their heroes provide buffs that really help khorne daemon/bloodbound in general. Problem is they don't tend to have any mages (they can unbind spells at least with some units) So they'll have to deal with Mystic Shielded units (+1 to Armour), and other such shenanigans based around mortal wounds or spells or other such trickery, not to mention sniping out heroes will weaken the army as they tend to be needed.