You and I have different concepts of a low carb diet. For me it's 30-50g on low days, and 100-150 on high days. If done correctly, your strength should never stall on even an ultra-low carb diet (<30g). If you are steadily losing fat on this diet, there is no reason to switch it. If your progress stalls, I would consider moving to something like 240g protein, 50g carb, 100g fat.
Running at 5-6 mph is definitely a little higher than what I was referring to. 4 hours of steady state cardio will definitely interfere with strength gains. For fat loss, it isn't even necessary. As far as good HIIT workouts, I would advise that you post in the conditioning forum. You'll get some better advice over there than I could ever give you. To be honest, I never really do any steady state or HIIT. While cutting, I use my weight training as my "cardio" by bringing rest intervals ultra low and performing high intensity full body circuits. With a program like 5/3/1 though, you will need to supplement with some sprinting workouts.