Richard,
I'm hoping that other members will chime in here, my experience and information is just one side, there are many sides to be had, so hopefully people will start posting.
I think however that you posted this in the wrong place... you should have posted it in the main nepenthes topic here:
https://forum.petpitcher.net/forumdisplay.php?f=6 that is where general discussion about nepenthes is, scroll down and you will see all the different topics, the first few ones you see in that page are forums like propagation.
Maybe the admins can help us move the topic later, meantime I will reply.
1. Gosh your request sounds like you are trying to make a scientific study of the compactness of potting mix. Which is all good, but unfortunately I don't think I'll have time to do it, because what you are being like is a Baker... very precise, while I am like a chef... all by taste and feel. Meaning, I don't really study the compactness, I just do what I do as mentioned above. If after soaking and draining the mix I find that the pot needs more potting mix, then I add more then soak again and drain again
Also as mentioned, different species live in different substrate in the wild, some live in clay like soil, some are epiphytes, some in bogs. So again it depends on the species.
2. Why did I stop using Sphagnum moss? Well first of all they are hard to find in my area (Philippines) very difficult to buy from stores, so I am using them only for bog plants, VFTs, droseras. However that is not the only reason. I find that if I am careless and forget to water the plant or plants, the sphagnum moss can dry out and when they dry out they need to be soaked again or else they tend to not be able to retain moisture. I have killed a plant because I let the mix dry out too much and didn't know about it, so even if I was watering the water would quickly drain out. So now I usually put even my neps under a water tray... and if I do this with Spag moss, the mix will be too wet and boggy and not ideal or nepenthes either.
So I use now cocopeat and coco chips, if the species likes a more moist and compact mix, more cocopeat in the mix or pure cocopeat. If it likes airy mix, then more cocochips. Sometimes I include pumice rock in the mix too.
Lastly please understand this is just how I prefer to pot my neps, I don't claim that this is the best or ultimate way to pot neps, but the system works for my gardening habbits and situation
TTFN
Arvin