You can. Just need a US address to get started. I use one in Washington that I have anyway for use as my billing address.
They will eventually cut you off for being mostly-roaming.
Are you sure about that? It's been almost a year since I got my mom set up with Fi in France, no issues yet.
There was a post on here (IIRC) about a month ago about someone who had family in the US, but his phone was always overseas. They killed the whole account.
As in the whole Fi account, or the whole Google account?
The Fi account, IIRC.
Not having a local number would be a bit of a pain.