Okay I’ll do the 4. questions first because I’m relatively sure I can answer them correctly. The SA node sends a signal via the atrial-ventricular node (AV) down the purkinje fibres in the centre of the heart, causing it to contract from the bottom upwards (so the ventricles can empty themselves before the atria contract to fill them up again). The SA node’s also known as the pacemaker since it sets the rate at which the whole system moves. (Incidentally it’s one of the few parts of the nervous system that works completely independently of the brain).
I don’t know how much you know about nerve signal transmission! Suffice to say if a sufficient amount of ion movement is caused by whatever stimulus it launches an action potential, and this will cause the muscle to contract. The AP is an absolute value (it’s either ON or OFF) so there’s no change. The muscle contracts simple if it’s switched on.
ACh (small ‘h’ so not to get confused with adrenocorticohormone!) slows down the RATE of APs – so how many times it’s switched on in a given period of time. Otherwise all correct, and just the reverse for epinephrine.

1. Stroke volume in the amount of blood pumped from a single ventricle and so is entirely dependent on the size of the ventricle and nothing else! Over time obvious a greater number of contractions would pump more, but this would be cardiac output rather than SV.

2. See above

3. Okay well the heartbeat is regulated by both PNS and the SNS. The PNS is associated with autonomic functions (so ones we don’t think about like breathing in and out) and via the vagus nerve controls the baseline heartrate of about 60-80. However during exercise etc. this needs to speed up to supply more oxygen and at this point a sympathetic function takes over and allows the HR to rise.

Yes congratulations on 4!

I’m afraid I don’t really know for 2. Contractions in the atria at a basal level are caused by the SA node (which is part of neither system), but it is affected by parasympathetic from the vagus to slow it down, and sympathetic to speed it up. To the best of my knowledge the vagus nerve is only involved in autonomic functions. Sorry I can’t be more help here :s

Heart rate increases when the body’s trying to combat change. So you’re quite correct in thinking that heart rate will increase in the cold as the body tries to warm itself (especially when shivering) but it’s also worth noting that it also withdraws blood supply from extremities to conserve heat.
There are two ways to look at cold: firstly the heart rate should slow along with all metabolic processes since it’s these that generate the majority of our heat. However the peripheral blood vessels all dilate to let blood closer to the surface to cool us off (hence we go red in the face when we’re overheating). This lowers blood pressure and means the heart has to take up the slack by pumping more. I think you’re going to have to use judgement here because it’s going to depend on the severity of the cold!