Be aware that once you open the scanner, presuming you can get past the
mechanical issues you are having, the pristine clean interior will never
be that way again. These things are assembled in a clean room so that
there is no dust inside the unit. (Dust = spots and defects in every
scan that you make). Working in a home or ordinary repair shop is no
substitute for the clean room and you will find it impossible to keep
the internal parts of the unit dust and lint free. Remember that the air
all around us is full of dust particles and it will get all over the
internal scanner parts despite all best efforts to "keep it clean".

You are perhaps overlooking the fact that the "internal" dust will be at
some distance from the focal plane.