Process of elimination. The EU rule provides for passenger compensation in cases where their flight (singular) cancels/is delayed/they're downgraded. The OP has at least 3 segments, probably at least 4, that make up the itinerary. It's just common sense that a problem on one segment isn't cause for 75% of the "cost" of the entire itinerary isn't due for compensation when only one segment is affected. What does make sense is that the "cost" of that segment is the basis for compensation under the EU rules.

Just think of IDB rules in the U.S. - you don't get a full refund plus penalty for the half of the itinerary that is completed satisfactorily. You get only the cost of the cancelled segment plus penalty. You have to look as it as any government rule - if it doesn't say something you don't get anything. The EU reg doesn't say "75% of the cost of the itinerary" so you don't get that. It says "75% of the cost of the flight" so that's what you get - if the OP and UA can agree on what that cost is.

Jim