1) If I have two equal strength grenzer units and no other units have more hit points, can I allocate 2 hits so that both grenzer units are partially damaged?
No, units with partial damage always take the next hit first.
2) Prussian has 2 units in the charge box and 2 units in the charge versus infantry box against 2 Austrian units in the charge box at the end of round 1.
Cavalry in a column cannot slide from one to the other. They must wait until the return to the form-up box. If at the end of a combat round cavalry are unopposed in their column, they immediately return to the form-up box.
I hope this is clear. Glad you're dusting off the game. I still enjoy playing it when I can, but there are so many new games coming out all the time that it gets overlooked.