Have torn ligaments in knee. Can femur actually slip off the top of the tibia? Could something fracture? Using exercise bike & walking for strength.

No. The femur would slip off the tibia if you major injury, also nothing will fracture unless you major trauma, having said that you stability is weaken by the ligaments injury so in your case less force needed to dislocated your knee or break the bone around the knee. Be careful and good luck.
Torn ligaments. It would be unusual for your femur to slide off your tibia unless you tore all the ligaments around your knee like in a knee dislocation. If you just tore your acl then your knee can be unstable with subluxation of the bones. Sometimes the acl can avulse from the tibia with bone. Exercise and knee strengthening is good for a acl tear and consideration of surgery can be discussed with your doctor.
Yes. The acl keeps the femur from sliding forward on the tibia. Its unlikely that it would fracture or slide all the way off, but even a little bit of instability is bad. Its like putting a tire back on a car and not tightening the lug nuts down on the rim - eventually that wheel will wear out prematurely compared to the others. Hamstring exercises help but surgery is recommended.
It depends. Your femur may not slip off comletely (i.e. Dislocate), but it can slip a little bit (i.e.Sublux) leading to torn cartilage and arthritis. Its like driving a car with the lug nuts loose on the hub of the tire. Eventually that wheel will get damaged and wear out prematurely from rotating improperly. Your knee will wear out prematurely too - that's what arthritis is.