Is Drupal scalable across multiple geographical locations?
I'll address your two points one by one and then discuss suggestions. This is a massive question in scope so I'm doing the best I can with what we've been provided.

WAN-wide cluster

Galera would be your best option if you wish to do an Active/Active setup. It's a tricky setup though and only the best hosts have figured it out (platform.sh comes to mind). Your other option is, by far, the most common. i.e. Master-Slave replication. You may find support for that in the Drupal community is more prevalent and that is worth its weight in database queries. :)

Content sharing between locations

You have some options when it comes to this. If your editors are to be collocated into a single geographical space, then sharing content isn't a problem. If you have a need to allow editors access from many geographical locations, you can do this in a two primary ways:


Active/Active using a Galera cluster as you indicated.
Active/Passive sites using a content deployment model using the Deploy module, Migrate module, Feeds module, or some combination of those.


Suggestions


Start thinking in terms of a single site, not sites. Ultimately, you'll employ a CDN of some kind to deliver content across regions. You had mentioned a concern with freshness of content but there are many ways to achieve the balance. i.e. Look to the Drupal modules Purge, Cache Actions, and Rules. 
Consider Edge Side Includes which will allow the static serving of some content on a page but assets served from the edge for others.
I can't stress the importance of dedicated Memcache servers enough.
CDNs, Varnish and a module such as Boost are not mutually exclusive concepts. They can all be employed to keep things fast on the anonymous side. Bear in mind, however, the more layers you add; the greater the possible complexity.


Hope this is helpful.