The plane would still be seen on radar, even with the Transponder turned off. It would not be broadcasting an identification signal in response to radar. That is what transponder does. It receives a radar transmission and reponds to the radar with a response transmission. Thus it is a transponder. It would be an unidentified flying object. They would have plenty of time to scramble fighter planes to check it out and shoot it down in necessary.
Not necessarily.
I've got a fair amount of experience in this area (B-52's, a long time ago). A plane flying at low level is very hard to spot on radar, especially if it follows the right route. Even if flying high enough to be painted by radar, the typical ground control radar would not necessarily spot it.
An AWACS could spot it, but only if one was in the air and in the right place.
However, a plane flying at low altitude will use a lot more fuel than it would flying at a higher altitude. Hence it is likely to have MUCH less range than you would otherwise expect.