I've never seen 2x4 trusses used for the bottom of the attic/ceiling
of the living space.

Trusses are almost always built with 2x4's, except for specialized trusses
like attic trusses.

Only problem is he said the attic has a floor and depending on how
much of it has floor, how many nails were used, etc., it could be
some trouble removing plywood to get the insulation in.

Did he mention an attic floor? I must have missed that.

If that's true, adding insulation will be problematic. Also, as you said,
the bottom 2x4 chords won't carry any significant weight, certainly not for
a living area in the attic.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com