Will Versus Be Going To

First of all, let me summarize the differences between will and be going to as typically done in grammar textbooks:
Will is used for
1. Decisions made as we speak: The pone rings. So you say, " I will answer it."
2. Predictions based on knowledge (not physical evidence): According to the weather forecast, it will rain tomorrow.
3. Offers and promises: It looks like your bags are too heave. I will help you if you want.
Be Going to is used for
1. Decisions made before we speak (say the decision was made last night and the next day the speaker talks about his decision): I am going to travel to Greece.
2. Predictions based on physical evidence: Look! It's very cloud. It is going to rain.

If that understanding is correct. Let's try to tease out "will" and "be going to" further by looking at this marriage proposal scenario:
Jack: Will you marry me, Jane?
Jane: Of course, I will.
The proposal and reply are probably considered typical English utterances. But what if Jane instead of typically responding with "will" wanted to say that she has actually thought long and hard before about Jack and has actually made up her mind about him and wanted to convey or emphasize that idea. She has been expecting this question over and over again and she never wavered about her response. She knew all along her answer.
Could she consent to the proposal by saying: Of course, I am going to marry you.

Well, this is a long winded question and might be a silly one and fantastical indeed. What do you think?