How can I identify the Influence Character?
How can I identify who my Influence Character is when he/she is also involved in the rest of the story as an Objective Character?
First of all, try thinking about your story only in terms of what is REALLY going on -- not what SEEMS to be going on. This is the viewpoint that most clearly identifies the storyform. After you know what your story is truly about, then you can hide it, hint at it, and otherwise obfuscate it from your audience. The events as they TRULY transpire make up the story's Plot. The events as they are presented to the audience is what we call Storyweaving. In works that rely on mystery and suspense, the storyweaving will present things much differently than the linear progression of the Plot.
The questions you should then ask yourself is this: What is my Main Character's PERSONAL concern? This issue is something that the MC would take with him or her even if the other characters went away. That will help define the MC's point of view. Then ask the question: Who in the story has a fundamentally different and alternative Point of View on the same type of issues. Identifying that individual will help you identify WHO your Influence Character is.
The alternative approach is to PICK a character as the Influence Character and GIVE him/her an alternative world view to that of the MC. Sometimes that works -- especially if you do not have a clear idea who the Influence Character is.
On your point about what the Main Character's personal issue is, if his or her problem is "Disbelief" AND he or she is a character that ultimately changes to resolve his/her personal issues, this would indicate that FOR THIS CHARACTER looking at things in a skeptical manner leads to conflict or errors (or other such problematic behavior). This would imply that the personal solution for this character would be to open his or herself up to belief in order to resolve his/her personal issues. If the character ultimately Remains Steadfast to his/her skeptical approach in an effort to resolve his/her personal issues, then disbelief would be better understood to be the source of his/her drive and not so much as a "problem."
Concerning the teacher as an Influence Character, the teacher need not be aware of the MC or his/her impact on the MC for them to act as the IC. Of course, this is more difficult to storytell, but it is very doable. However, the IC must represent an alternative attitude or approach (paradigm) to that of the MC when considering the issue that is key to the MC. Having a different pov is not enough. It must be a different POV on a single issue -- the issue that is pivotal to the story in general and the MC specifically.
Let me reiterate about being "objective" about your own story. For the moment, dismiss the storyweaving from your considerations. Determine what is really going on in your story. Then, and only then, re-look at the Dramatica questions and answer them based on this purely Author's point of view. You should find this process a little easier, and hopefully determine the answers to the questions that are nagging at you.