We're on the same page when it comes to the Elly/MC seduction that Jenna's not needed but ok you're not a fan of threesomes.
Using what you're saying then there would be no natural arc for anyone to have a threesome because, although Ada, Emily and Elly love each other unconditionally, the thought would never cross their mind about making love with each other. Elly's too timid to do anything else apart from service the MC and the MC would be too uncomfortable to suggest a threesome seeing how unsure Elly is being with someone else rather than the MC. Jenna understands the full range of MC desires because they're been there already.
If, say Ada or Emily walked in on Elly and the MC, and despite the MC trying to convince them to try it out then and there, it'd feel forced that they're doing it for the love of the MC because they'd be jealous of each other and hesitant with each other. Going from no thoughts to full on threesome would feel forced.
I get that a 'traditional' path would follow what you're thinking 1) Seduce everyone make them unconditionally do anything for MC 2) Create situations where one of them 'accidentally' walks in and watch what happens 3) Explain situation with the person who walked in and plant seed 4) Experiment and enjoy. Doesn't this different path make it more interesting?
Not exactly the direct sister but Where the Heart Is has something similar.
EDIT Just read a quote by
@RomanHume that might explain our ways of thinking