Great questions. Thanks for writing in to us.
I think that some of your confusion comes from our description of ME work in the two books. In TftNA ME for mountaineers and alpinists was our focus. In those activities you will be moving steeply uphill with a heavy load quite slowly. The tempo will be slow but the perceived exertion can still be quite high. If the load is high enough your perceived exertion can feel like Z3 or even 4 in your legs while you breathing and HR remain more in the Z2 range.
If the loading is appropriate in a ME workout then the HR will be significantly lower that what you might guess if you were not looking at your HR monitor. The limit to your speed is the local fatigue in your legs not the global fatigue that you would experience if you were unladen and running up that same steep hill.
It is a matter of matching the load to your individual needs. I have had athletes with crazy strong legs who needed 50% or even more of the body weight to elicit this training effect. I’ve also seen 20% of BW bee too much. With your background you are probably going to need a heavy load.
It can be tough to find steep enough ski slopes to do this workout. The skins will slip. For ski mountaineering I recommend the gym ME program described in TftUA.
I hope this helps.