The way I see it, source is just a more general statement of a source. It could refer to the geometry and often does, or it could refer to everything about the feature unless otherwise specified (by another *:source tag. Maybe using a single source tag for multiple characteristics of a feature is suboptimal, but the alternative can be very repetitive and possibly obscure the most relevant use of the source.
For example, when I sketched in this house, I gave some newspaper sources mainly to establish the house’s existence and dates. The geometry was an educated guess based on the negative space between buildings and some local knowledge – I used to pass by the house every day and only learned of its historical significance after it burned to the ground. If I were to tag it as source=Minh Nguyen or source=Inferred from surrounding buildings in Esri while crediting the newspaper articles only as name:source etc., I think that would be deeply misleading, as I’m no expert. Instead, crediting the newspapers in source would center them as the most important sources, and if I really want credit, then I can put it in geometry:source.
I think this approach simplifies your model somewhat: there’s only one kind of source key, but the unqualified one is an umbrella source for the feature, which can be further refined by more specific keys. There’s very strong precedent for this approach in OSM’s source:geometry and source:position keys (other than the reversed syntax).
Yes, Wikipedia has a similar standard for citing maps, even for citing OSM. There’s also a full complement of non-map citation templates, some of which have been ported over to the OSM Wiki. Over time, I fully expect that non-map citations will outnumber map citations in OHM, so we’ll need more robust tools for managing citations.
Some Wikipedians swear by the built-in Citoid tool or the third-party Zotero service to manage possible citations while conducting research. Nothing stops a mapper from also using Zotero (which is standard-issue in academia). You could use it to generate all the metadata tags and paste them en masse into iD’s raw tag editor.
Let’s just keep it simple for now. If the wiki has a bibliography page for common local citations, stick the URL of this page in source or whatnot. If you pass in |ref=harv to a wiki citation template such as {{cite journal}}, it’ll automatically produce a named anchor that you can include in the source tag to point to the individual citation. Then you can use source:id for the pinpoint. Here’s a live example.
Later, we could add some HTML microformats to these templates for machine readability or move the citations to data items for easier querying. This is all doable today with minimal effort, but I think we’d want to get a better feel for the requirements around citations before stubbing it out.