RE: SIZE 2 strength
1. Routine looks ok, you've got deadlifts, dips, bentover rows, and squats in there. Just one thing, on your second day, all three exercises are chest exercises, drop one of them and do close-grip bench instead for your triceps.
2. Good question, you could do it every 2nd/3rd week instead of deadlifts or squats.
3. plate curls, wrist curls, thick bar work
4. 3x5 is fine as long as you increase the weight appropriately from 5x5 so you're still working with the same high intensity.