Hi Maggie. Thanks for responding. I will definitely look into this book. I am especially interested because photography is what i do so there is a double draw here.
I will try to address your questions, but first some qualifying statements. I am hardly an authority here. I was drawn to the path of the drum over 20 years ago and it has led me many beautiful places. Drumming is very often a ritual experience for me, but i have never had the opportunity to drum for a Haitian style ceremony. I have drummed with the New Orleans Voodoo Temple on a few occassions as well as the Niagra Voodoo Shrine, but the rhythms used by these groups are completely different from the Haitian ones i studied in NYC, which are much more complex. The change of rhythms with these groups is not seamless in my experience and the Mambo made it perfectly clear vocally that we were moving from one Lwa to another. AFAIK, in Haitian ceremonies the Mambo or Houngan signals the switch, usually through the ason.
I can say without question that changes in rhythm undoubtably do change the energy of a space. The rhythms for the different Lwas often have very different character to them, a different vibration.
I have drummed ritually in many different circumstances and traditions. As i think we are all aware, the Neo-Pagan movement has pretty much absorbed afro-percussive practices into their own and i have done much drumming in these realms as well. I can say that where ever i have been where the drums get cooking and the dancer start moving Spirit arrives. Whether these are the same spirits as the Lwas is debatable. Sometimes i am sure they are. Sometimes they are something else all together. It does seem to me that the more specific the rhythms are the more likely it is that specific spirits will arrive. There is definitely a different feeling one gets when playing a specific part in ensemble. I can even feel this when playing these rhythms in the context of a dance class where no ritual is intended. The free-form drumming of Neo-pagan communities can't always guarantee the same results like a set ensemble piece like Yanvalou or Makanda can produce. Still, drumming and dancing even in these improvisational forms can still raise enormous power and draw in the spirits.

I am sure i will get some arguments here, mostly from people whose experience is outside the realm of Vodou drumming, but it is not considered at all good if a drummer is ridden and a possessed drummer will quickly be taken out of the ensemble. Trance drumming is a shamanistic technique usually done with one drummer doing lone spirit journeying or perhaps in a Native American style where a group of drummers are all playing one large drum on the same down beat. It is very effective in these settings, but trancing while playing poly-rhythms is not IMO a very good idea if you want to keep the rhythm together. This is not to say that i do not feel an energy flowing through me when i play and it is sorta like a high, but i never feel like i need to "come home". When i am playing i am "home". ;-)
Grounding afterwards is not a bad idea though. Often just lying on the earth for a few minutes if all i need. And drink lots of water, all the way through.
I have played all parts in ensemble drumming, that is to say bell, boula, sécond and maman (lead). In Haitian rhythms generally the boula sets a steady pace (often on the up beat), the sécond provided the basic rhythm and engages is "conversations" with the maman. The maman plays the lead patterns and breaks.
Now in open drumming experience when i am playing djembe i often take the lead, though it is best when there are other experienced drummers that you can trade off with and have conversations with. But this idea of "leading" is complicated really. Sometimes the drummers lead the dances, but sometimes it's the other way around. A good drummer learns to watch the dancers and "mark" the dancer's steps, sometime anticipating them.
In Vodou drumming the lead drummer also does this. It is, of course, the job of the ensemble to bring on possession. When a lead drummer sees someone on the verge of possession they will usually call the break. The break is a radical change in the character of the rhythm and it is devised to push the person on the verge all the way into possession.