This is good information so far. Thank you!
There are only so many variables I can account for realistically, so I have to accept that there are some things that just need to be allowed to slide in the name of getting some information (instead of none, which is what we have now). Some of the considerations that you're talking about are good in theory to keep in mind, but somehow I doubt that the average healer is thinking quick enough to go through all of those permutations each time they make a healing decision. BloodSim isn't intended to model inhumanly perfect play.
Though it does sound like I need to have something that will create the effect (as in, a gap in healing received) of one of the healers having to periodically switch tasks and heal someone else in the raid, because that's really the only aspect of that action that matters from the perspective of keeping the tank alive.
I think you're on the right track if all you're looking to do is gather information about the approximate interaction between absorbs and the blood mastery.
As for how to decide when to use the three heals. This is a function of the rate of incoming damage versus how long the tank will live without heals (because most heals have a cast time). A healer usually isn't going to cast a heal that heals at a slower rate than the damage the tank is taking. The main exception is when the higher throughput heal has a cast time that the tank would die within. Then you might use a faster heal with lower throughput (and probably higher mana consumption...think fast inefficient heal) until your partner(s) can wind up a large heal on the tank. This sort of thing was pretty common in BC, so I imagine it happens more in Cata than it did in Wotlk. I might add for clarity that the rate of damage the healer is considering upon the tank has the amount of healing provided by the rest of their team subtracted.
All healers will have a preference for their medium heal over the slow or fast heals due to mana concerns. Certain heals like penance and holy shock will also be higher in priority for the classes who have signature single target heals like those.
Enrage/Decimate phases are also important. The healer has to know that they need to pump out the max single target healing they can during the enrage phase (assuming appropriate gear for the encounter) and they have to be prepared adequately to do so. That is, they need to have conserved and/or regened enough mana to make it through the phase. During such a time, you probably don't have to consider the prospect of a healer switching to another target. Perhaps getting cc'd or something, but not switching.
Decimate is something like the reverse. The healer has to know that they need to get everyone
topped off asap, especially the tank, but the tank is more easily stabilized than during enrages. Here, healers would be swapping targets much more frequently as the tank is usually least likely to die since a decimate is usually not accompanied with heavy tank damage to my knowledge.
You might consider a healer's decision to cast a regen ability that lowers their output (channeling, or divine plea ms for example) as well. Basically, they'll only do it during a lull in damage and/or if the rest of the team can manage for a few seconds.
Also don't forget things like pain suppression, guardian spirit, lay on hands, and the like.
I'm sure I'm missing things, or some people might not agree with stuff I've said, but I hope this helped. The variable with the largest effect on the output of your sim will probably be which healer(s) has been assigned specifically to heal the MT.