Indirectly answering your point #3 above, this would appeal to inter-country travellers. More specifically, there is probably a sizeable segment of customers who travel back and forth between Canada and the US where this would be a hit.
At $19/month, it's also cheap enough to be a work/travel phone that is assigned to you. Although there is a growing trend for IT-consumerization and multi-SIMs, there are still plenty of people who prefer to keep physically separate personal and work mobile devices. This could easily appeal to them and organizations.
For a smaller (but very lucrative segment) of roaming subscribers, being able to use a single phone number accessible over WiFi no matter what country you're in is a compelling value prop; no more need to worry about data/voice add-on packages.
It's like being able to take your voip line with you on your phone, accessible through one number. Surprisingly, nobody else can offer that right now. The closest we've got is FaceTime (cellular network for voice; free voice calling if you're on wifi and reaching out to another iOS device), and even that doesn't quite provide the same convenience.
And with declining voice revenues, carriers are fighting a losing battle knowing their margins are being squeezed. They are deathly afraid of voip cannabilizing their voice revenues.
This is a good move by Sprint. I wonder if they have any equity in Republic Wireless.