Looking at your 4 and 12 leads. Nothing pokes out as "STEMI" other than the LP12 interpretation. I understand your concern for the elevation in v1 and v2. But consider you QRS width. It is greater than .12. I would question whether or not those wide QRS complexes are physical contraction. If PVC's then no. The elevated ST segments are only noted on those wide complexes. That's inherent. I would not trust calling a STEMI alert based on the monitor. Except for the occasion of new onset LBBB. But we would need to dig into the axis variations which are for the most part WNL. Maybe off slightly, nothing to indicate infarct however.
I notice you list a history of NIDDM. Safe to assume non-insulin dependent diabetes? Did you check a blood sugar. Frequent syncope with disorientation can be a red flag.
Either way, ASA and NTG administration in the field with a suspected head injury is a NO NO. If, the closed head injury you suspected is in fact that, and secondary to the fall, ASA would effectively remove/restrict the body's compensatory mechanisms of coagulation. And NTG, being a vasodilator, we would increase the blood flow to the injured site; worsening the injury.
You were right for with holding. Good job on cervical stabilization. You have an injury above the clavicle with AMS thus an unreliable patient. We do not know if there is or is not a cervical injury. Good job erring on the side of caution. A spine board would be pushing it, and really not necessary.
Careful with high flow oxygen. A proven vasoconstrictor and if we have an injury, in the head, could cause hypoxic problems near/around the injury site. Low flow is sufficient given the PVC's. The heart and brain are connected more than one may think. It is quite possible the closed head injury you are suspecting is causing the PVC's.
There are several STEMI imitators out there. I applaud you for considering all possible diagnosis'. Did you follow up to decipher the outcome?
But no, no STEMI to be noted.