I spent a whole day just trying to figure out what was wrong with my code on the site. Worked fine in dev, but when pushed to AppEngine, it just flat out broke my API calls. FWIW, if you want to get a virtual IP on AppEngine (which fixes the 'problem'), it's like $40-something a month extra.
That's what we ended up doing. It's expensive per month, but you can roll up multiple sites under a wildcard cert (I think -- I'd have to ask the devops guy who is thankfully dealing with this mess).
Yes. The criteria for avoiding SNI is one IP per cert, not necessarily one IP per domain. If you can consolidate subdomains under one cert, you only need one IP. The issue that SNI deals with is with cert switching on a single ip:port... not anything inherently related to multiple domains on an IP.
