Sometimes a bit of blue can show up in the auto-map (or mini-map) when the entrance is hidden behind a wall. I don't think hitting the alt key will highlight them, but if you move the mouse cursor back and forth across an area, you should notice the mouse-over description when you find one.
There is one summoning doll per act. I pretty much only used the act 1 doll, though, mostly as a mule, a couple times as a trap detector, and occasionally as support in tough fights (using a crossbow somewhere out of direct combat). I had 2 dolls at the end of act 3, as well (didn't think to try to avoid having what's-her-name restored).
I saved before doing what I was warned not to, and briefly had a pink DK. I preferred black, though, and there was no great reward to compensate for the colour change, so I reloaded.
The after game BF consists of a smallish temple building filled with rifts. Each time you enter a rift it generates a new BF (one of the act 1-4 BFs).
The act 3 and 4 BFs have Blood Knights. There is a Death Knight in act 4, though you do not necessarily have to fight him.
