The original plan for 1897 was probably for it to end with Quentin dead on the chair & sealed in the room. When he became so popular, they had to come up with an alternate scenario.
They really didn't, and it's always seemed strange to me that they thought they had to. There were plenty of changes they made to the "time-line", many events which un-happened, such as Q never having been a ghost. Yet, in that BC-JH-QC conversation the day after Petofi's having saved Quentin, Q asks "Then whose skeleton was in that chair in 1969?" or something. And instead of telling Q that this has now never happened, along with all the other things in 1969 which have now never happened, Barnabas says he doesn't know, but that there is some explanation for the skeleton.
There didn't have to be. What after all explained the ghost that looked like Quentin hanging around 1969 Collinwood? Did that have to be a ghost imposter, since they now know Q survived? No. It just didn't happen in the DS world... "anymore".
If there was a solid reason for their feeling compelled to come up with an alternate explanation for the skeleton (rather than just time-travel naivety on the part of the writers or DC), then I suppose it was that they'd already thought of a cool and ironic means to end Gregory.