Is a controlled fractional crystallization process feasible on the Moon?
First, read my highly related answers to your other questions:

https://chemistry.stackexchange.com/q/67403/8083

https://chemistry.stackexchange.com/q/72483/8083


  The forsterite might make decent clear glass


Probably not, see above.


  fayalite can be processed with hydrogen to get iron, water, and silica, and the silica can be used to make glass


Again, probably not. To make the fayalite component react fully in reasonable time you are going to need to mill the olivine to very fine dust. Then, separating the silica from this fine dust in solid form is almost impossible. This will require heavy liquids (like polytungstate) which are not available on the moon. Doing this in liquid form and centrifuging will require heating, which will inevitable cause reaction of forsterite and silica to form enstatite: $\ce{SiO2 + Mg2SiO4 = 2MgSiO3}$, which is useless for glass making.


  Is a controlled fractional crystallization process feasible on the Moon?


Basically yes. This would be similar to a distillation process that slowly over repeated melting and solidification processes will remove Mg-rich solid from Fe-rich liquid. The problem is that even very low amounts (1% and lower) of Fe in forsterite will colour it green, which is undesirable for your glass. And that's without even considering the forsterite is not a good glass material (see links above). Seperating the Mg out will become harder and harder as you go along in your process.

Good glass material has alkalis in it. Fortunately, some lunar rocks have Na and K in them (e.g. the KREEP basalts). It has about 1 % of potassium, which could be enriched in fractional crystallisation. The problem is that you will also enrich the glass in Th, making it slightly radioactive and possibly browning with time.