Unique Key find MAX Date w/out Going Over Another Date

I work at a school. We have leads that we try to convert to a student, the funnel is lead, application, and student.

I'm trying to find the campaign where the student transferred from application to student, which is called the primary campaign. The systems is technically set up to do this automatically based on the below; however, I'm finding that a sales member is going in and adjusting that primary campaign.

I have attached an example and in column AA shows that I want the final answer to be. This is a massive spreadsheet of multiple leads and dates they were leads and dates they were applications.

Essentially these are the parameters: The application date must be less than the student start date and the closest application date to the start date. Column X "Was Application" needs to be a 1. Because there are times where there is a start date associated to it, but they didn't finish.

You can see that in column I, there is a 1 on the top row right next to the primary campaign source of Sales Generated. Based on the dates for this specific lead, it should be a "Webinar" not "Sales Generated".