I'm not an ex pat, but I have dual citizenship. I've never lived or earned money in the US but I still have to offer my income and declare my bank balances to the IRS every year - the principle is enough to make me want to return my passport, never mind the paperwork.
We do exactly this on one of our products where I work. Geographic projection with our tiles up to zoom level 12 or so then switching to Mapbox mercator tiles beyond that. Assuming the styling is consistent, the change is more or less imperceptible.
A few years ago I worked on a digital library system and we would sweat over including proper metadata about articles in meta tags (derived from the Dublin Core attributes entered by the authors). That was until we met with someone at Google who worked on Scholar. He said there was no harm in including that metadata, but they usually had more success inferring things like authors, title, etc from the HTML content of the page so they ignored it for the purposes of indexing. Things may have changed, but basically I think you're right that semantic markup is easy to game so probably doesn't affect SEO. Doesn't mean it's not worth doing though :)
Blank is purely for validation and has nothing to do with the database constraints. If you want to prevent empty input just remove 'blank=True', surely?
I'm not an ex pat, but I have dual citizenship. I've never lived or earned money in the US but I still have to offer my income and declare my bank balances to the IRS every year - the principle is enough to make me want to return my passport, never mind the paperwork.