You didn't mention this, so I'll also suggest you consider taking shots in your "butt" or upper hip area. The more you'll consider rotating injection sites, the better overall absorption you'll enjoy and the less "abuse" any one area will feel.
I know what you mean about avoiding areas that HURT HURT HURT! I never did find a good region on my legs, but while I have skinny arms, I can take shots there easily. The place I use is best described as what "flaps in the breeze" when an outa shape person stretches their arms out parallel to the ground. That underside seems just designed for insulin shots ;-) I either push my arm up against a wall or else contort my body a bit to reach that area.
Good luck ... I have had diabetes for ~35 years. While I now pump, I have no scarring from years and years of MDI (multiple daily injections). I still take shots from time to time when I want to quickly treat a high blood sugar.
When my daughter was on shots we and even now when we change her site, we put a drop of peroxide on the site , a l very tiny dot of Neosporin. Depending on the location a band aid for 24 hrs (she is 8) she has no scarring or marks since we started this, occasionally we do get bruising, but that usually goes away with in a week.
I have done some research on your question, and have not found any answers to the possibility of speeding up healing. However, as a type 1 diabetic who is also very lean, I have some suggestions for you from 34 years of personal experience with diabetes and insulin injections.
First, I would encourage you to use arms and legs for the actual shots in addition to your stomach in spite of your lean body build. The arms do not bruise as badly for me as legs, for the large blood vessels run on the INSIDE of the arm rather than on the top of the leg to possibly get knicked by your needle, causing bruising. I have also discovered that you can inject into the fat areas just covering BOTH muscles in the upper arm, not necessarily into just the upper area. Even a thinner person is OK injecting here, and the further back on the arm you go (almost on the back side of the arm), the less painful it is. Also, a thinner person can inject into buttocks, for even thin people have a little fat tissue here. You gotta be able to reach around, though. So I would encourage you to add other sites for injections. You will do less overall damage to your fat tissues by allowing some rest in between sites.
Also, as far as speedier healing, here are a few suggestions: if you find a bruise happening (you can usually tell because it is painful), try placing ice on it for about 15 minutes. This will keep the blood vessel that was punctured from seeping more blood under the skin, and will keep bruising minimal. Also, after the first day, if you find that a bruise DID occur, try using hot packs on the site for about 15-20 minutes to help speed up circulation to assist the body in washing that bruise blood out of the site. You could do this in the evening as part of your preparation for bed.
I have not heard anything about Preparation H as a bruise aid... perhaps someone else will comment about it.
A note of encouragement for you -- I have been injecting insulin for 34 years and have no scar tissue visible anywhere. Bruising is more of an issue, and eventually you figure out where you will bruise and where you will not. And bruises don't scar. So you probably have little actual risk of permanent scarring. You can stay proud of your lean tummy!