i guess fox mandated a 4 act structure some point during season 20's production that excluded most holdover episodes except this one because maybe it was early enough along to change it?

Yeah, it's really odd. It's the last episode in production order, so you'd think they'd do them all the same? My guess is that Fox demanded all the episodes in the new year that way, and that happened to be the first episode of the new year

either that or fox changed up how they aired things starting in 2009 and that episode was the first one to air that year during the midseason break