This jogged something in my memory and after a little googling I sure enough found this article from last year, where the Russos list off which characters were killed in the snap off-screen. When asked about Jane the response was "too spoilery".Works for me. Perhaps Jane spent the five years after the Snap trying to locate and reassemble the missing pieces, with or without Thor's coach potato guidance.
That anything at all with Foster was considered spoilery certainly raises some red flags and that this is year old information at this point probably indicates that some kind of agreement, or at least an interim plan was already in place when they made 'Infinity War'. Of course they also say in that article that Aunt May wasn't dusted, so make of this what you will.