Hi ltjbukem, welcome to Flyertalk!
Assuming you're not flying in business class, I wouldn't count on admittance to the Iberia lounge.
As for the Diners Club lounge in MAD, I popped my head in there erroneously when I was looking for the Iberia lounge, and it looked very large and pleasant (at least from the lobby area!) At least with regard to amount of space, chairs, etc, it looked like a perfectly pleasant space in which to spend a few hours.