1. I would recommend to space enough time between the sessions, to allow for the specific adaptations.
2. not really. But you should be careful with max strength training at your age. And often HR is higher when you are younger.
3. the chest straps are usually accurate. Measuring from the wrist not really.
As 14 is pretty young, I would recommend getting a good coach/mentor, who can guide you on your way to training and enjoying sports and the mountains. There are some mistakes you don’t have to make yourself;-)