You fellas have access to some very affordable hydraulic components. And like sawdustdad said, these are toys, not production machines.
Definitely a float valve would be an asset on the blade height cylinders. I could be wrong, but I believe you could add one after the tractor's original valve.
Just operating the cylinders on the grader won't cause much heat. A bigger issue with using the axle housing as the reservoir would be, IMO, the fluctuation in fluid level. You've got several cylinders in the system and each one has a volume differential between the two sides. Depending on rod diameter the difference can be quite substantial. The difference between having all (or most) cylinders extended or retracted at the same time would be fairly extreme in the small axle reservoir. There would be times of underfill and times of over fill.
To hydraulic drive the front would absolutely require a separate pump, motor and reservoir. And would be near impossible to synchronize.
If I was intent on having a front drive, I'd change the rear drives over as well. I'd remove the hydro drives. Take two salvage motor shaft/pinions and mount a motor to each. Drive those motors through a three port flow divider with a separate pump direct mounted to the engine. Figure the difference in the driven gear ratios between the rears and the donor front drive and get a motor to suit that.
I think a better investment in time and engineering would be a walking axle.
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