Glad to hear that things seem to be running smoothly so far - what's the BA error message? Quite strange that you can check-in for the IB flight (was this on ba.com?) but not for the ones before, as you normally have to check-in in sequence...

I'd like to stress my earlier point of making sure you will receive a wheelchair at the gate upon arrival in Madrid - they might see you board without one, and assume you don't need one. The crew, upon opening the door at destination, often inform/confirm with the ground staff the number of wheelchairs needed. The Sin Barreras service in Madrid is quite swift fortunately.