[nradov]

That depends what you mean by "low". The F-35B (like any fixed-wing aircraft) is perfectly well flyable down to stall speed, and below stall speed it doesn't fly at all. For the conditions described, stall speed would probably be something close to 120kn (exact performance parameters are classified). (Some fighters can use thrust vectoring and other aerodynamic tricks to retain a limited amount of post-stall maneuverability but that doesn't really apply in this case.) In an engine failure situation, pilots are trained to trim the aircraft to the speed that will give the best glide ratio and that's going to be well above stall speed.

Vertical flight mode wouldn't be usable in a situation like this. They only transition to vertical flight at low altitude with the landing site in sight.