The light is connected to three switches that control it and two other
lights in the room. One 3-way switch is in the house, one 4-way by the
back door and one 3-way in the garage. Previous HO pulled another circuit
from the subpanel in the attached garage to the box so that the fan could
be operated independantly of the lights. When I took the fan down to
replace it, I discovered this.


Like it or not, I think you should "undo" the previous wiring UNLESS the fan
you intend to use can be rewired to separate the motor neutral from the
light neutral.

If you just have conventional CBs or fuses, it will work regardless of which
neutral you use but the "hot" and the associated neutral are supposed to
stay in the same cable.

Indeed, each cable regardless of use should carry ZERO net current. That
means the "return" current is the same as the load current. Unless you
have some kind of leakage to ground, proper wiring always ensures this will
be the case. It's even true with cables that only serve a switch and 3/4
way switch arrangements.

But that's mostly a theoritical concern. What might cause "complicated"
problem would be if someone "for safety's sake" replaces on or the other
breaker serving your fan with a GFCI breaker. The odds are great that the
mixed or combined neutrals on single pole circuits will cause all kinds of
false trip problems.