I should have mentioned this earlier but I am assuming you have ADS.
So my understanding was that I could jog while in Z2 unless the terrain or elevation causes me to go out the zone and therefore walk.
Yes, you are correct.
Does this mean that I should exclusively walk the hills I’m currently getting about halfway up when jogging before getting out of Z2 just to stay in Z2 the whole way round?
You will have to figure out what works best for you. If you walk the entire section, does it help to keep the heart rate in Z2? Then do that. If a jog/walk combination works better to keep heart rate in Z2, then take that approach.
If the top of my Z2 is 150, do I wait until it drops to 135 or is somewhere in the middle okay?
As you progress you will figure out what works best for you based on the terrain.
I just want to say that it’s okay if your heart rate goes above or below Z2 for a short period. These zones are not exact by any means and as far as the majority of your training is in Z2 you are good.