This worries me a little because a MC has the best or worst answer. Thus, an IC has the best or worst answer. These are absolutes. I suppose we could have three answers and one is good (bad), one is better, and one is the best in a particular scale (scope). Sometimes, there is no great answer but there is always a best answer in the given circumstances.
Looking at the definition of IC...
If we look at the definition Every Main Character has a single Influence Character that forces him to face his personal problems.
In law, it is dangerous to only look at the statute. Many times Case Law makes a clarification of statutes that weren't clear legally or that didn't take into account subtleties. Statues are necessarily broad. This allows maneuvering in the future via Case Law.
But, if I also look at another part of the definition of IC:
In every act, a story problem is introduced that requires the Main Character to expose his personal problem in order to solve the story problem.
I read this as saying that each Act has a single manifestation of the MC's overall problem.
Artistically, I could see each Act allowing for a different IC.
Artistically, I could also see a group representing an IC. For example, I could see three characters representing the IC if they represent Mind, Body, and Spirit. Or four if the ICs represent the different Classes.
I'm planning something similar in my novel. As long as one aspect isn't redundant, I don't see a problem with making a group of characters an IC as a whole. Dramatica seems to be (IMHO) about finding balance -- as in a balanced argument.
I feel more comfortable with having two or more ICs if they form a greater whole or they are the Change characters. If the MC is steadfast, then you view him as the IC in their (partially told or implied) stories. In this way, there is only one IC.
If you look at art, there has always been a clash (a good thing) between form (structure) and content. I think that this is part of the fun in the fields of music, poetry, etc. Yes, we follow structure, but the Greats aren't afraid to bend and break the rules whenever necessary to achieve their goals as artists? What do we fundamentally try to do? We try to create an emotional response.
I think, as an audience, we can intellectually get into our own way when we are dealing with emotions. As artists, we many times want to create visceral responses rather than intellectual ones. But we have to deal with intellectual responses. We have to clear the road for the audience.
To borrow another term from filmmaking, Dramatica allows the Diegesis of a story to continue by keeping the observer, reader, or consumer from being pulled out of the narrative. In other words, it removes the potential for disbelief or rejection of a story due to objections over redundancy, logic, realism, etc.
What is the base function of storytelling? To create emotional responses that aren't fettered by intellectual shackles. That's not to say that intellectual shackles are bad. They aren't. We have a term in storytelling (suspension of disbelief). I feel Dramatica allows for and encourages this suspension of disbelief. It helps to eliminate the "but what about..." syndrome unless we want the audience to ask that question.
Caveat Emptor: I am less experienced than you in regards to Dramatica. As always, these are just musings on my part.