A: Yes, but watch your thermals. For basic floor plans and elevations, it is fine. For rendering or massive files, the Air will throttle (slow down) because it has no fan.
A: No. Autodesk has "Revit Viewer" in the cloud, but you cannot create or edit models in a browser. You need the local install. The Bottom Line You cannot download a native "Revit for Mac" installer. But you can run Revit perfectly well using Parallels or Boot Camp.
Don't panic. You are not out of luck.