1) Almost 2 years
2) Few errors lets say once every five-six months mainly in brokers plug in. Hard to say if was zorro's or broker fault. Some bugs in Zorro's broker plug-in was found during this period.
3) Cannot compare. While using Zorro's and MT4 as execution platform both platforms performs well under stability point of view. MT4 is, without any doubt, more resources hungry.

Said that I believe that MT4 is intrinsically more stable under the trade execution point of view. MT4 is a client server application and the interface between the broker and the platform are managed by the broker IT department. Usually well structured to provide service to a community of MT4 Traders.
With Zorro's totally a different story: size,sometimes matters!

HtH