That choice was definitely diappointing because even though we are eventually going to have start making choices like this, it's not like the MC couldn't have said, "That's sounds great, it's just that I promised a friend I'd check up on them. They're not feeling so good so I thought I'd go see how they are and if they needed anything. But, hey, do you know what? It shouldn't take long, so give me half an hour and then we can go out." Realistically, he could do both, check on Sage and go on a date with Jill, but this is an unfortunate consequence of game mechanics that it forces an otherwise illogical choice here just for the sake of it.
Yeah, it sort of boggles the mind that Jill expected the MC to leave immediately instead of assuming she would need to coordinate their schedules. I don't think she knew he had the whole day off; they haven't spoken much during the week, but I can't remember for certain if the topic came up in the library. But the dilemma just vanishes if Jill could wait an hour, so c'est la vie.
That said, it's a lot less problematic if this doesn't actually foreclose future paths with Jill. I'm not keen on the odds (given how irrationally Jill is taking Tybalt's ultimatum), but there is at least a chance MCs previously on the Jill route could still have a shot. Presumably she will eventually explain the whole Tybalt situation to MCs on a friendship path with her, so the topic should come up again... someday.
On a similar note, I'm now a bit worried about the concert. It was unpleasant watching Jill struggle with the piano when she refused to call the MC. Does this mean some of paths will never get to play that song on the guitar for her, or is that going to be the excuse to defy Tybalt and kick off whatever the next phase of Jill's story is? I wish I knew.
I think if you're on the friend path with Sage you don't get that option. You need to be FWB with her, so that would be kinda awkward bringing Jill along.
No, you definitely have the option to tend to Sage even if you turned her down.* The hangout is the same but it ends early, with Sage just falling asleep instead of cuddling with the MC first. But the MC does have to choose to call Sage, it isn't a mandatory task the way it is for FWB.
*Well, as long as you agreed to give her guitar lessons, and maybe called her in Episode 5; I've never tried snubbing Sage completely, so I don't know if there are specific triggers.