We've got veins returning blood to the heart on both sides of our body, and it all has to go via the circulatory system - not as-the-crow-flies from any point directly to the heart.
In any case, even if there was (seems not) a few seconds delay in terms of how quickly poison from a snake bite on one side of body vs other got to the heart, that's not going to make any difference to survivability.
It seems that developmental asymmetry, resulting in (fairly major) left vs right brain functional differences, is what has lead to left vs right handedness.
I would say that brain asymmetry probably is not the cause of hand and leg asymmetry. It has to do either with venomous [1] plants and animals or with muscle use and fatigue.
When the hand is stung or burned it performs a quick centrifugal movement instinctively. Then many mammals can suck some blood out with their mouths. That minimizes the venom in circulation, and some additional time to get to the heart might be less damaging, especially when the venom is not very plentiful like bees, scorpions etc.
When the venom is of a big quantity, like big snakes then probably it doesn't make a difference.
One other factor is muscle isolation and helping the heart to not get tired from work. Considering that the whole point of gyms is muscle isolation, which is very difficult to achieve normally, a human or monkey cracking nuts or cracking bones for hours, it will tire the left side (where the heart is located) much more increase palpitations, and consequently the risk for heart attack.
That last one could be tested, working more using the left hand -> increase palpitations or not. As a counter example, the chimp when it cracks the turtle, it works with both hands [2].
Your reasoning makes no sense. Using your left hand to crack nuts vs your right hand makes no difference as far as your heart is concerned. If you are working hard enough to increase your metabolic rate, then your heart will be pumping faster in either case. Also, your theory that increased exercise leads to heart attacks is the opposite of reality!
Exercise prevents heart attacks when you have excessive calories, you get adequate sleep and do not sleep outside in the winter, you have a dry place to stand and are not out in the rain and so on. Exercise leads to muscle loss when the fat reserves are depleted and calorie intake is limited.
Anyway there are ways to falsify or verify that hypothesis, but it needs some resources, not much but still, it needs time and subjects to test the hypothesis on. So for the foreseeable future, it will stay as it is, a hypothesis.
In any case, even if there was (seems not) a few seconds delay in terms of how quickly poison from a snake bite on one side of body vs other got to the heart, that's not going to make any difference to survivability.
It seems that developmental asymmetry, resulting in (fairly major) left vs right brain functional differences, is what has lead to left vs right handedness.