The line is that Portuguese can understand Spanish, but Spanish can't understand Portuguese. Portuguese Portuegese does indeed sound Slavic. I'd always thought it was, till I worked in Portugal. Brazilian Portuguese is about halfway between Portuguese Portuguese and Spanish, to my ear, anyway. I can understand it more or less from knowing Spanish, whereas I lose Portuguese Portuguese instantly.
So Lamela would probably get the gist of what Paulinho and Sandro say in Portuguese, but not 100%. They would probably understand his Spanish more or less completely, unless they had issues with his accent. I used to talk to an Argentine woman and her daughter, and could understand the daughter more or less completely, and the mother more or less not at all. She had a way of forming her consonants which I found so distracting I would lose what she was saying right away.
I'm sure AVB would understand Lamela's Spanish perfectly, as he knows Spanish fluently, I believe, and could speak to him in Spanish as well. If he decided to speak in his Portuguese, on the other hand, I'm guessing it would be Greek to Lamela.
As Sidney says, the Argentines are known as Latino Italians, due to so many coming from Italy, and the Italian influence on their accent. I still wonder if Lamela could be introduced to Argentines or Italians who would help him make himself at home here.