With regards to project documentation, "the plan" could refer to two things:
- The document outlining the plan
- The plan itself, in action
For example, a particular diet may be outlined in a document such as a book. You would speak of details being in a book, yet someone who was following it would be on the diet.
Likewise I believe you would speak of details being in a written plan (such as a project document), but once the plan was in effect you would speak of things being on plan to mean that you were still following the plan, or things were going to plan.
In your other example where "plan" is used to describe something like a mobile phone contract, you would almost certainly say "on" the plan if you were referring to anything inclusive in it:
"Would I get inclusive text messages on this plan?"
But in your example you specifically talk about "add-ons", which by nature are extra to a plan. So if you were asking about anything extra that may be bundled if you took a plan, you might actually say:
"Would I get any add-ons with this plan?"
Because the the add-ons are not part of the plan, but you might get them as well.
Your last question:
"There is talk that Airbus could snatch even more market share in that deal. They have made a killing on the deal"
Refer to the first part of my answer. If you are referring to the plan as an entity in itself, either the document or the concept, then you would say something is in that plan. But once the plan is up and running then you are on that plan.