Multi gang switch boxes - Is it okay to keep the same branch neutrals separated for the different lighting/switching circuits?
Nice drawing. Very clear and looks correct. It would be a mistake to tie the two neutral wires in box A. This would create a circuit loop and a fire risk from heating when one circuit is on and the other is off. There may also be hum if you use switches or lights that include electronic circuits such as fluorescent ballasts. It's OK to wire this up as drawn.

Green/bare grounding wires should be treated in the same way. If there are two separate grounding conductors feeding box A (e.g., NM cable) then they should be tied separately as drawn for the two neutrals. If you feed box A with two grounding wires it is best not to use a metal junction box. If junction box A is metal it needs to be bonded to one of these grounding wires and will thereby be bonded to both switches. In that case you can't avoid a ground loop. This would not be a safety hazard as these wire carry no continuous current but it may cause hum as noted above. So use a non-metal box and run NM cable or individual wires in NM conduit.

Better yet, remove the standalone A-to-C run by running HTTN(G) from box A to B as well as from box B to C. Then there are no neutral or ground loops to worry about and you are free to choose the most convenient wiring method.