Nothing about making Cavendish is critical, except safety. As with cooking food, there are many different ways to do it, yet still create a result you will enjoy.
Look at the size of your pressure cooker, compared to the size of your Mason jars. See how many will fit at once, and still close the cooker lid. Pressure cooker max pressure and max temperature depend on your elevation above sea level, so don't give it much thought. If your cooker will fit 5 jars, then make up a jar each of 5 different varieties. They all come out different. Cavendish cooking does indeed smooth the taste, and soften the aroma. But if done correctly, the cooking has no effect on the nicotine level of a specific variety.
If you cook in a jar sloshing with water, then you will leach out both nicotine and flavorants (i.e. make bland Cavendish), unless you allow the finished leaf to reabsorb all of the water, prior to drying the leaf. Lots of water does make for darker color, if that is what you are looking for.
My preference is to tightly roll each frog-legged, wet leaf into a snug sausage, and pack as many as will fit into my chosen jar. I do not add additional water, though a bit may drain down from the rolled leaf. Add the lids and rims. I add all the jars into my pressure cooker, then use a small dinner plate to add weight to the top of them, to hold them down, allowing me to add a greater volume of water, before it makes them float and tip. If they can tip over, I add a small, capped Mason jar of water, as a space filler. I run the pressure cooker at its standard pressure (~15 psi), which will cook at ~121°C (~250°F), for about 5 hours. But you have to attend to it, to make sure you don't run out of water.
A less worrisome approach is to set it all up the same way, but with a different pot lid, so that it is just a pot of regular old boiling water with a cover. I can then peek every few hours to check the water level, and if needed, add boiling water to the pot. With this approach, I just start it in the morning, and let it go all day. I don't leave it boiling over night. Sometimes I'll even cook the same batch for a second day, depending on my whim.
It's worth noting that the final color of the dried Cavendish will not develop fully until the leaf has been removed, spread out to dry, and well exposed to air. (Even commercial "black" Cavendish is actually a deep brown, if you evaporate most of the chemicals from it--glycerin and PG). Once the jars have come out of the water bath, and allowed to cool, I flip them over (stand them on their lids) from time to time, until any residual liquid in the jar has been well absorbed into the leaf.
Since the leaf needs to be unrolled, and fully spread out to dry, each jar of leaf will occupy 2 or 3 square feet of drying space (cookie sheet or cutting board). So I usually open only one jar, and dry the leaf, while leaving the remaining (well sterilized) jars sealed. One generous batch of jars of Cavendish may take me 10 consecutive days to dry--typically 24 to 48 hours per jar full. Once dried down, I usually just bag the dried (low case) Cavendish varieties, and don't shred any of it until I'm in the process of making a pipe blend.
Bob
EDIT: I should add that the aroma continues to evolve for a week or so after it's been bagged.