Im not sure which place should the grenades been shoot, certainly there's an inconsistency anyway.

The position of the flash is correct. The weird thingy at the end of which a flash appears is the AGS-17. The design of the launcher was a bit reinterpreted by BI but that's exactly where grenades are supposed to be fired from, not through the main cannon.

You can clearly see the actual thing on this picture:

there might be an engine limitation to this

I'm afraid I don't quite follow 😕 From the looks of it the muzzle flash appears at the right place. It isn't supposed to appear at the muzzle of a main cannon as the rounds aren't fired through it but through the AGS-17 launcher mounted to the top of a turret.

i mean that the game only accepts one set of points for bullets and shells to be shot from, which means both autocannon rounds and grenade rounds can only spawn from a single point in the 3d model, rather than both having their individual spawn points