None. Inhibition of the vagus nerve (which slows the heart) disinhibits the heart rate effectively speeding it up. Atropine is commonly used for this purpose.
Vasovagal response. Vascular dilation from excitement, more flow less rate, can be variable.
When I start jogging at low speed, my heartrate shoots to max heart rate of 173, until after 5mins it normalizes to 123. Is this normal?
No. Your resting heart rate should be around 80 bpm, 120 is very high, you also have a exaggerated heart rate response to exercise, this could be due to being deconditioned, or if you are using certain medications, drinking excessive amounts of caffeinated beverages. I recommend you see a doctor.
Yes, but. It's not a safe way to get the job done. Being riddled with bullets would make your heart rate fast too (for a while) but obviously not the way you want.