Hooray! It looks like Safari 12.0.3 finally fixed my running problem with Safari 12! This is the first Safari 12 update I tried that didn’t introduce the .webloc crashing problem (see my 11/21/2018 entry above). I tested with both a fresh Sierra (10.12.6) install, and my fully up to date copy of Sierra that I had downgraded back to Safari 11.1.2. In both cases updating to Safari 12.0.3 did NOT introduce the .webloc crashing that I encountered after every previous Safari 12 update (12, 12.0.1, and 12.0.2).
The curse has lifted!