This is an interesting idea.
If I may add to this hypothetical (Sam joining the museum visit) I think it would work well to progress Sophia/Sam, Sophia/Patricia without cheapen up the evening event with "some strangers" that are not bound to come back.
- Sam Event
L&P sort of confirmed that it would be a cooling-off event, with the main focus on cello practice. As we speculated, we can expect Sam to try some moves on Sophia during practice (groping her clothes or sliding his hand under her shirt and so on), Sophia will calmly put him in his place, there may be a kiss, but she manages to defuse the situation.
Then the sketch Sam drew the day before might come up again and they start talking about his skills and art in general. And that's when Sophia might ask him to join in. And of course he does.
- Museum Event
Let's assume that everything will be "consumed" inside the museum. Obviously Jason and Haru might not be thrilled that Sam came along but that's life. They can start visiting the different sections of the museum with the 2 college students doing the honors and guiding them through the different pieces of art. At some point when in the painting section Sophia and Sam might start to fall behind, caught with the discussions about the meaning of the painting, probably with another rant against modern art etc... 
After a while they try to catch up with the others but they are nowhere to be found. Only when Sophia goes to the restroom, she finds out that Patricia is "thanking" the two students in the bathroom. From the half-open door she starts to peek (the usual cliché), gets aroused and her hands starts to wander through her body. Suddenly she feels Sam's hands on her body while he starts to kiss her neck... You get the gist At some point the sisters meet their gazes (similar to Patricia in the shower during the yoga event) and they continue their moment of pleasure in a complicit manner.
This scenario would prepare them for the next day at the ranch for what we all hope will be the long-awaited lesbian event. It would progress a bit the story with Sam (the focus remains on Sophia, no BJ etc). It would disclose their relationship to Patricia who could play the usual important role in making it progress: during Sophia's cello concert event in front of their parents (PD 17) she can e.g. "distract" the others while Sophia and Sam get some time alone.
Of course the major problem is that it would tie the museum event to Sam path which is not probably what L&P had in mind at the beginning. But, hey, speculating is free, so...
I thank you for providing your perspective (precise and accurate as always), but I don't believe that if, eventually, Sam were to play a role in the evening event on Day 21, it would be only a marginal role (no bj and focused attention on advancing the relationship between Sophia and Patricia in preparation for the lesbian event the following day).
So as they say in this case... I see your speculation and raise with my own fantasies.
IMO, L&P, especially for the evening events, has by now gotten into a pace (as far as the progression of Sophia's sexual experiences is concerned) that can hardly be interrupted with one event in preparation for a later one.
Certainly there could be an interlocutory first part in which, for example, the two sisters might challenge each other in teasing their young companions, but once the visit is over, they should move on to do something more interesting (hence my assumption of placing Sam in this event, since after Dylan, he's the only character with whom Sophia has gone that extra mile that might warrant further progress).
As already said, the event could follow the now established pattern of a two-part division with two different locations.
In the first part (the softer one of the museum visit) I speculate that (similarly to what we saw in v.0.195) we might be asked to play not only, Sophia's character, but also Sam's (and/or Patricia's) character and make choices that would influence the course of the second one.
In the second part (the one most interesting from the point of view of erotic content with the drive home with Sam), on the other hand, Sophia could pay back all the favors her young and caring neighbor has done her so far.
Thus, once they arrive in the driveway of the Parker house, this sort of reward for his services could begin by providing a visual stimulus for Sam to finish his evening without "thinking" too much about her (Sam of all the charms Sophia can offer seems to be very attracted to her breasts, and a teasing like the one Aimee did in this last update could serve the purpose very well). Then, once she saw Sam's response time lengthening, Sophia, in order to get Sam to come as soon as possible (and thus not make anyone suspicious of the time spent in the car with him), could switch to manual stimulation (as with Dylan in v.0.190). Finally, if Sam were still "stuck" by the unexpected situation (and only corresponding to choices made earlier by the player that I cannot tell you now), Sophia, mindful of her performance in her high-school days, glutted by the situation (Sam's might be the first non-family member cock that Sophia has at her complete disposal since moving to San Alejo) and under the pretext of not wanting to ruin her makeup or clothes with Sam's sprays (in this case, it doesn't pay to emulate Monica Lewinsky), might quickly switch to giving him a blowjob and (as any good cheating wife would do in this situation) swallow every last drop of what he would have to give her. Then, once re-dressed, Sophia could run home to her husband, who would be waiting to hear about the evening at the museum with her sister and his friend Neil's son (it would be funny IMO if he even kissed her on the mouth... "Mmm, you taste fishy - did you have a taco before you got home?").
So it might start the usual hypocritical pondering of the limits she has crossed and the guilt that goes with it (but then, with each update, those limits move further and further until, hopefully, she starts to give a fuck and indulge in lust and passion). After this, probably Sam (not sure when - before or after Sophia's performance in front of her parents) will be busy with the school trip and will be out of the game for some time (at least from the point of view of the events that concern him as a co-star).
This could be a scenario (I don't know how plausible) that could lead to Sam overtaking Dylan again in terms of hot scenes with Sophia during the course of the game (maybe in response to what might take place on the afternoon of Day 21 itself between Sophia and Dylan himself - someone speculate about a boobjob).
You know, there was a time when I used to think that we would have to wait that, more or less, every co-protagonist (at least the most important ones) would have to move on from the experiences had by the others in order to advance the story (and with it Sophia's experiences) and that then a handjob for Dylan (v.0.190) would have to be matched by a handjob for Sam (in one of the subsequent updates) and so on until all the available slots (Morning, Noon, Afternoon, Evening) of the remaining game days were occupied. But now, especially after the evening events of Days 19 and 20, I believe that from now on, at least between these two Sophia's LI, we will attend, even more than before, a real race (made up of mutual overtaking, sudden braking, and relative off-roading) worthy of the last laps of the
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between Villeneuve and Arnoux (too bad, now as then, I suspect it will just be for a second place).
