Gee! so sorry,
I also think 15 ft is excessive. I try to stay in the 8 to 12 ft range and still 12 ft is a little far. If the house is old and has no plywood on the walls, the walls can rack and sag. The diagonal bracing is pretty good but does not compare to modern building techniques. Perimeter walls on modern construction usually have a continuous foundation wall under them. The foundation wall that runs perpendicular to the run of the floor joists is the major load bearer, but even so, the walls that run parallel with the floor joists usually have the gable wall on them and are bearing a great weight also.
It may be time to call a certified building inspector to evaluate the work done and abide by his findings.
Tom in KY, a building inspector can sometimes be better than an attourney. Inspector D,, Are you out there today?