I use the Bentonite up front and have made it as a slurry as the instructions suggest. Another mead maker that I respect has suggested it is possible that adding it dry may provide less of an impact on flavor vs the slurry. I have been adding it dry for about a year now and appreciate not having to make a slurry.
While the Bentonite does a fine job of dropping yeast and other itty bitty things out of suspension, I find that the raw (sometimes unfiltered) honeys have enough pollen or other items that just like hanging around even after the Bentonite. Bentonite works best and clears the fastest when used up front like in a wine kit.
It is best up front with fermentation as the CO2 bubbles will carry Bentonite to the top and it will grab stuff along the way. When the CO2 bubble pops, the Bentonite will drop to the bottom and grab more stuff on the way down.
Depending on how "dirty" your honey is, you can use .25 to .5 teaspoon per gallon of Mead. I have some very raw Buckwheat that will not clear on its own without fining agents. That required Bentonite and then Sparkolloid to get it clear and the flavor of the mead was much improved as there was just so much junk in suspension.
At this point I'll add oak/vanilla/whatever after I rack off the Bentonite and leave that for however long I need. I back sweeten if needed/desired and then I add Sparkoloid. In as little as 8 hours, the difference is immense with regards to clarity.
I will cold crash it as well at this point. I haven't tried cold crashing before racking off the lees and Bentonite.