I read the question very carefully. She does NOT say she wants an authentic milonga. She DOES say she wants real bandoneon musicians (first mentioned requirement), indoors, seating, top-class dancers (non-acrobatic) and good food. Catulo fits all those criteria. So do many authentic milongas (it's debatable whether those of us who attend these are "top-class dancers" who visitors would want to watch, but I'll leave that to your own self-assessment), but it's fairly hard to get bandoneon musicians at those before midnight, and, because I read the question carefully and saw the visitors are "elderly", I thought of Catulo, which starts at 8:30pm. It's a good option. Entry tickets to it are raffled at a milonga generally regarded to be in the top two or three remaining authentic milongas and winning one of those raffles is how I got to know about Catulo.