I never said the individual guns need to perform the trajectory calculations. Just that the possibility exists with a computer by the gun. Whether each gun should do this or let the battery do this is a matter of doctrine for each military and not technical capability.Your later explanation is long, but fundamentally incorrect. It is not necessary for an individual gun to perform these calculations, these will be done by the battery command post. Once gun position is properly recorded, command post can then simply bark out the bearings/elevation/propellant charges. All the additional sensors would merely provide more accurate data for firing solutions.
Similarly, command post can relay the firing coordinates as desired by the forward observer to the individual gun crew. Digital firing computer is totally not necessary to program a guided round. You can radio the coordinates and manually punch it into the fuse setter and then program the round.
So why build in fancy electronics into the gun if you can just use guided rounds on a cheap gun? Of course it is expensive in the long run to use so many guided rounds.
I don't know what fuse setter you have, but the fuse setter that the M777 uses doesn't come with a keyboard to punch in target coordinates with. It needs to be hooked up to an external electronic device to receive the data from. Could that external electronic device be portable and come with a keypad/keyboard and receive digital data from the battery? Certainly yes. At the very least you could show a picture or diagram of the system you are talking about?
I should clarify that the computer associated with the howitzer (not necessarily attached directly to the gun) can also be used for C2 purposes (i.e. receiving and responding to digital orders). The purpose is not necessarily just for collecting and sending data from the instruments on the gun.
Tracking barrel wear and manufacturer discrepencies in the propellant and projectile using a muzzle velocity radar, temperature sensor and other "electronics" applies to both guided and unguided rounds.