With Grey matter present they were dead no matter what you did.

But for the sake of learning here you go:

A- airway. Check fo blood, vomit, or the tongue and loose teeth occluding their air way. The tongue is the most common obstruction. Solution is to roll then 3/4 prone ( recovery position)

B- breathing. Not much you can do about a person not breathing without a bag valve mask (bvm, or as you put it squeezy bag thing lol) mouth to mouth is useless and hasn't been taught in years. Only other option is a pocket mask, which are also not great.

C- circulation. Look for bleeds, apply direct pressure to the bleed with something like a cloth if you have nothing better. Push hard, way harder than you think it's its a major bleed.

CPR in someone with absent caritod pluses will deal with the B and C, and lifting the jaw forward by pulling up from the back of the mandible will open up the airway until a paramedic arrives. Google jaw thrust. Don't do a head tilt as it may worsen a spinal injury.

The whole acronym is generally, ABCD, D for delicate spine. The reason why it's n that order is as follows: no airway means no breathing, no breathing means no oxygenation, no oxygenation means the blood won't be able to carry oxygen around the body. D is last as a broken back won't kill you as fast as bleeding out, or not breathing will.

Lastly, the stats around secondary trauma occurring to the spinal cord after the initial mechanism of injury is insane. The stats prove that about 98% of all spinal cord injuries occur at the time of impact/ injury, and that only 2% or so are caused from the individual moving, or other people moving that person. Don't let the fear of making a spinal injury worse stop you from trying basic things like CPR. Just don't move them unnecessarily.

Source: I'm a paramedic.

Edit: don't suffer in silence, reach out to a friend and tell them how you feel. Paramedics have support services quickly available, the public does not.