1) I see that the 'nightly builds are up to v5.1.4x. Are we close to having an "official" 5.x release? or close to having a projected release date?
We are working hard in order to get MonoX v5 released as we need to abstract almost all modules and cross-cutting functionality in order to support IoC, also we are adding new WebParts that will make MonoX even better. We are hoping to release v5 officially in the late September.
2) Once released, would such an "official" 5.x release be able to upgrade a site running any of the previous nightly builds? or only 4.x instances?
All v5 based sites would be easily upgradeable while upgrading from v4 to v5 will still require some effort, as soon as we have v5 ready we will publish a detailed upgrade guide.