While the lockout on your strip heat at 35 degree is a good idea, I really doubt if this will solve your problems. And with all those extra BTUs from being oversized, I would lower the lockout even further.
Yes, your HP can go into defrost mode with an outside temp of 48 degrees as well as other existing conditions. I think I would want the defrost function checked out on this system. Defrost at this temp usually is unlikely.