A few "tree" thoughts in this "forest":
Well...I can't "fault" her thinking..that would be MUCH easier, yet it still doesn't trump changing the "character" of the trip between sisters bonding...validating that girls can go do hard things in the world "without a man"...and maybe they "do without" for a couple of days and become better planners as a result..or just become more resilient by working through the hardship of "doing without".
1. I'd let go of the "it'd be better for D24 & D15 to learn that they are capable" angle. It's likely true, and it's something you value. It's in a different position in both their hierarchies of values than in yours, though. I think they know where you stand by now, and it'd end up getting you all, as CF says, in the weeds to try to convince each other that "the trip would be better with/without a guy along". So, given how many issues could get woven together here, and your desire to be deliberate and clear, and your limited energy, I wouldn't spend any more time on this angle.
2. Especially because I think the issue isn't whether the trip would be "easier/better/harder/whatever" with or without BF. It sounds like one of your core values is that regardless of anything else -- regardless of whether D15 got great grades & told the truth about them, how nice of a guy new BF is, if he sleeps in his own tent, whether two or three horses are perfect loaders and well trained... regardless of any other variable, you would still say No to D15 going if the only other two people on the trip are D24 and BF? Is that an accurate distillation?
3. I re-read this line:
I still struggle to come up with any good reason I should get onboard with other people going.
So just to make sure... if it were D24, her best female friend, and D15, that would also not be OK with you? Or is it specifically the BF? Really want to make sure I understand.
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I agree with empath:
I think it would be wise to separate the discomfort with the ride from the grade issues. Combining them "muddies the waters" and could make the whole thing confusing
and CF:
I know you don’t want to get into the weeds with D24 about some of these issues. Suffice it to say that perhaps buying a new horse without doing adequate due diligence is not an idea you will support.
Keeping different issues in separate lanes seems like the way to go.
If, no matter what, it's a hard No for D15 to go on a trip where there's one other person plus that person's adult boyfriend... then it's a No, no matter how perfect everything else is in the setup.
One thought I had was that if you end up saying to D15 "No, I'm sorry, you can't go on the trip because an adult boyfriend is going" (or however you phrase it), and she returns to the "you're punishing me [for the grades thing]"...
you could clarify that you wanted to keep the grades thing and the horse thing very separate, and you were planning a separate "consequence" for the grades, but "if she really wants to be punished for lying about grades by not going on the trip, you are open to that".
I mean... maybe don't really do that. But it is an amusing double bind. But please don't actually do that, unless it can be done with love.
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Overall, thoughts are:
jettison any extraneous issues.
keep separate issues in separate lanes.
identify and stick to one core issue/value around which decision-making pivots.