Prior to stamping, try to clear your skin of acne (or at least partially) with our 0.5% Tretinoin cream. Initially, Tretinoin may make acne worse
before it gets better. Apply it every second day for several weeks. It will make the skin red and maybe peeling.
I was afraid you have a tendency for getting scars because normally, acne should not result in such raised scars. That is why you have to be very careful with rolling/stamping and you must do a test on a small area of your skin to see how it reacts.
Do you get scars easily also elsewhere?
First, try the dermastmp somewhere on your body (for example on a thigh) to see if it heals OK. Then try it somewhere on your face. Then stamp one of the scars just once and observe, whether it heals OK. Gradually add more stampings into the scars, when the previous session went OK.
Immediately after stamping, perform the pressure massage (explained in the link I have posted here) and apply Tretinoin cream (it will likely sting).
When you stamp the scars, pull the skin upwards (away from your neck), it will make it much easier to stamp.
Buy a 1 mm dermastamp.
Concerning the allergy, you have to try. The contact with the skin is very short with stamping. Allergies are more of a problem with long term contact, such as wearing a ring, watches, earrings etc.
There is some nickel (the most common allergen) in stainless steel but it is tightly bonded so it doesn't get loose as it does in some golden jewelry.
Your question number 4 is answered here:
https://http://owndoc.com/dermarolling/how-dermarolling-works/