One option would be to use attributes together with a custom citation formatter addon. This could either combine attributes or just use free-form text.
We only have a two layer citation model at the moment (three if you count repositories as an extra layer). Citations are aggregated by source when they are displayed in the bibliography at the end of reports.
I agree. I planned to include “URL” and “Date Accessed” fields in v5.3, but we ended up making a fundamental change to our object storage format instead.