I've spent some time trying to fill that "gap in my understanding of how 'gut' dopamine is produced, and how it is used". I can't say that I've got very far yet, but there are a couple of things worth mentioning.

Firstly, I spotted an error in my first post. When I said "It is the dopaminergic neurons of the substantia nigra (and the cholinergic neurons of the locus coeruleus) which can not [tolerate the presence of alpha-synuclein aggregates]", that should have been "It is the dopaminergic neurons of the substantia nigra (and the locus coeruleus) which can not [tolerate the presence of alpha-synuclein aggregates]".

Secondly, I came across an interesting research paper, which seems relevant [1]:

"The principal components of the enteric nervous system (ENS) are two neuronal networks, the myenteric and submucosal plexus (SMP) ..."

"A recent autopsy survey indicated that there was no global or dopaminergic loss in the myenteric plexus in PD ..."

"Our results indicate that there is no evidence of dopaminergic and noradrenergic neuronal loss in the SMP in PD, thereby suggesting that neuropathology in submucosal neurons is unlikely to be a causative factor for GI dysfunction in PD."

[1] Appraisal of the Dopaminergic and Noradrenergic Innervation of the Submucosal Plexus in PD, Corbille et al., J Parkinsons Dis. 2014;4(4):571-6.

Appraisal of the dopaminergic and noradrenergic innervation of the submucosal plexus in PD. - PubMed - NCBI