I feel like I shouldn't be answering this for multiple reasons, but I also feel compelled to say I think it sounds really rough.
I don't know, I know my kid is especially sensitive, but I can't imagine him being away from his father for more than a day or two (and he's actually closer to me than to him). I don't even know if he'd be OK for a single night without DH in the bed with us. Even short separations are really hard on him.
I think maybe what you are asking is the developmental difference between 15mos & 2.5? While the day-to-day separation anxiety seems to be higher around 15mos -- when just stepping out of the room is a problem -- the overall anxiety seems to be a little worse at 2.5, which is how old my DS is now. At this age, he has a strong sense of our family, how things go, our routine, and really thrives on familiarity. Maybe that was always true, but it's much more apparent now, and he's really able to verbalize his anxiety, which makes it both easier & harder. He also is still getting a couple of the 2-year molars, and is just starting in on the 3-year-old nightmares/fears/phobias and a new round of clinginess. I actually think an extended separation at this point would be harder than one at 15mos (traumatic, actually), and I have a feeling many of my friends with 2.5yo's would say the same.
I hate to be so negative, maybe some will chime in with a more positive outlook! If it were me, I would keep the separation as short as possible (or not separate at all), even if it means cutting the trip short.
If you have to shorten your trip, would it be possible to have his family visit you for an extended period of time so she can bond more with them but be home and with DH around?