According to fire control system manual for export MiG-29, HMS coordinates which are measured vs HMS sensors on the dash(what's on the helmet is basically marker lights) are then translated into KOLS coordinate system which is based on aircraft's construction axis. All weapon cueing data is fused, and missiles are agnostic of mode in which they are cued. So it's all parallel and mitigated by missile's FOV, and gun is just close enough to the axis of the aircraft. Simple as that, no complications with convergence.
Кош's post in Helmet Mounted Sight & R-73 seeker head - LoS (line of sight) & aiming was marked as the answer