Re. Sandwich adapter. Vy nice. I’ll order one. that will simplify some of my complexity.
1/2NPT is a large fitting. If you can find a 1/2NPT to 12X1.5 adapter fine, but don’t let the adapter at the OEM port control your hose size. Use 1/2" hose and if it takes 2 adapters to neck it down to 12mm, so be it.
You shouldn’t obsess over things. Don’t be a whackjob. Moderation. In all things, moderation.