I'm wondering if these and fascia stretches will be effective for scar tissue from shots...
Not really - Ischemic compression will work well though. - the lacrosse ball is good for that. In fact I hadn't started to use the lacrosse ball until I started juicing (did I just really call it that?). There is another tool and I don't know the name of it but I will surely find out. It is an arced piece of metal that is used by a chiropractor or massage therapist to break up scar tissue in a muscle. Really though, any kind of deep tissue work, will help.
So after your shot, rub the injection site to work the oil into the muscle. Later that day, next day or couple of days later, break it up with the ball.