Don't panic. You are not out of luck.
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. Autodesk Revit For Mac Student Download
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. Don't panic