Congrats!

I wouldn't worry too much about the speed of the injection: I have mine done by a senior nurse at my GP's surgery, and she has 22 patients who get the same shot so she's very experienced. She says that if the vial has been warmed up in advance it should take about 30 seconds to dispense the whole thing, but if it's cold it takes up to 2 minutes. That's a top tip, by the way: always warm the vial up to body temperature before giving it to the nurse - it'll hurt less and it'll go in more quickly. Also, if you can, make sure you take two vials of Nebido in with you. The nurse has to aspirate the needle before she injects the shot, and if she hits a vein she has to abandon the whole thing. That second vial would mean that you could still get your shot. (I take one vial out of the box and warm it up, and leave the other vial in its box in my car, just in case).
You say there was another stain around the blood stain on your boxers: what kind of stain? Was it blood, or did it look like an oily patch? Nebido is a thick, oily substance, so if it looked like you'd spilled cooking oil on your boxers, you might want to ring Dr Curtis and ask his advice. However, it doesn't normally come out of the injection site because it's injected deep into the gluteal muscle.
And no, you're not allowed to inject it yourself; that's part of the licensing conditions for Nebido. It has to be administered by a trained medical professional and you have to hang around in the surgery for at least 5 minutes after your injection, because there's a small risk of Pulmonary Oil Embolism.