Unfortunately as the IB flight was delayed rather than cancelled, and you didn't end up taking it, there could be an argument to say you aren't due any compensation at all - see the CJEU decision in Laudamotion.

Would be very harsh for IB to reject the claim on that basis as you were still ultimately delayed by more than 3 hours, but I wouldn't put it beyond them as they are pretty poor when it comes to handling EU261 claims.

And yes, unfortunately it's IB you have to claim from since they were the operating airline of your booked flight.

Similarly you would legally speaking have to claim back the downgrade reimbursement (75% of the fare paid, which will include the Avios but exclude taxes) from IB. They may direct you to BA but that is not correct - as the operating airline, IB remains responsible. I imagine BA will only reimburse you the difference to the economy redemption cost, which would be quite a bit less than you're due.

Sadly there's no CEDR equivalent for IB so you may have to escalate your claim to the Spanish equivalent of the CAA. They're not exactly known for being speedy.