Re: Low heart rate (driving me nuts)
What you described could all be down to being unfit. You've had multiple tests as you've mentioned, so it's unlikely to be anything that can't be addressed with lifestyle changes. You're young enough in that you're highly unlikely to have too much wear and tear on your heart, so dropping the weight will make a huge difference.
Your heartrate dropping and shooting up on standing sounds like a circulation issue, along with the limb coldness. This can be caused by long periods of inactivity, as the heart suddenly has to push a bunch of blood to all of your limbs when you start moving around. If you have extra weight, it has to work a lot harder. It's also possible that you're hyperventilating due to anxiety, and this is making you feel short of breath along with the skin colour changes.
Chest pain, dizziness etc can be from anxiety over realising your heart rate is "too low". You've done the right, and sensible thing by getting it checked out, and since they didn't find anything worrisome, that makes anxiety a more likely reason.
What did they test your blood for just out of curiosity?
__________________
“Life is like riding a bicycle. To keep your balance, you must keep moving.” - Albert Einstein