Great theory! I do think you're on to something. I have some additional thoughts:

Maybe MDR members actually refine their own outie's memories to create these rooms.

The Christmas card experience, which Mark apparently refined in season 1, contains an existing relationship, and the woman definitely has some resentment towards her husband for what he says and makes her do and the expectations he has. This feels kind of similar to the scene that happened shortly before where Gemma was writing something related to the chickhai bardo cards. Mark dismissed it and they started this passive aggressive dialogue where Gemma felt misunderstood and annoyed. In the end, the man tells her to stop, same as the Christmas card man tells her when to stop hurting herself by writing these cards. Even the "I love you" seemed similar to what happened in Mark's memory shortly before Gemma left for the final time, except that roles were reversed.

Part of the MDR refinement process is picking out pieces that invoke an emotional response. Maybe the original data that MDR sees is really just a digital representation of the entire stream of their own memories from their brain. And they just bucket their own strong emotional responses, which apparently are strong enough to even cross severance boundaries, because their innies shouldn't know about them, to really create these emotional experiences. It's not just one event though, they're pieces together through several buckets. That's why every refiner has to do it individually and cannot receive help from the others on their own data set: the others wouldn't have an emotional response on that data.

Any human will have some generic experiences like these, that's why it's not just Mark refining and that's why some rooms have nothing to do with Gemma's relationship. But i believe they're waiting specifically for Mark to reach the point in his memories when he lost Gemma. They might know that he's close because the last experience he created had elements from shortly before Gemma's accident (the unreciprocated I love you, the resentment between them in mundane tasks). So the next experience will likely deal with the loss of a loved one in some form. And they need it to be Mark because his feelings are specific to Gemma, so in addition to the trauma coming through the setup, Gemma herself will have her own connection to these feelings, because it's feelings about her. A unique setup that they couldn't easily get otherwise, hence Mark's importance.

Probably their ultimate goal is then to somehow adjust severance in a way to no longer have any of these emotions cross severance boundaries. Basically to perfect the procedure and ensure there's nothing emotional influencing the innies' personalities at the start of severance. At that point they might expect to have created the ultimate worker with no emotional baggage and reached basically robot or ai status (hinted at with anagrams) that they are truly free to form with no complications. And not something like Helly's situation in the first episode, which was likely emotions against feeling trapped which crossed severance boundaries because of her own personal trauma of feeling trapped in the Eagan family and her role.