3) OneBusAway includes a REST API for accessing transit information. This is the same service that powers the OneBusAway iPhone app and will have data for not just King County Metro as other agencies like Sound Transit, Pierce Transit, and Community Transit are added in. See http://code.google.com/p/onebusaway/wiki/OneBusAwayApiReference
5) There are a number of projects already using the OneBusAway REST API, mostly targeting native apps for other mobile phones. If you are looking to do a mobile app for a specific platform, you might consider joining forces with one of these proejcts. See http://code.google.com/p/onebusaway/wiki/OneBusAwayApiProjects