Not a C36 owner, but could you glass a dam into your bilge where the shower drain enters and add a separate bilge pump in the new section created? That would keep the shower waste from the general bilge.
As far as the icebox drain goes... this can really become a noxious smelly bit of drainage - since you need a trap and inevitably foodstuff gets into the bottom the stuff that 'sits' there can get quite gross. Putting that into your bilge can be another problem. On one of our last boats we simply drained the icebox into a bottle that we emptied periodically and that seemed not too onerous a task. To lighten the load we used contained ice when we could (frozen in milk cartons, for example).
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)