Because of lacking SNI support? You can try an Ajax request or loading a picture over SSL and then redirect with JS if it doesn't fail. If the SSL site serves HSTS headers, all SNI supporting browsers will in the future default to HTTPS independently of the kludgey redirect (because all browsers that support HSTS also support SNI).
Sure. An unconditional redirect would be better, but that requires a dedicated IP
Sure. An unconditional redirect would be better, but that requires a dedicated IP