For seedlings in particular, Sunshine Advanced Mix #4 would be my first choice, add earthworm castings and it is perfect. Followed by FF Light Warrior, Happy-Frog (for clones) or even plain Sunshine Mix will work.
Materials make a difference though, they aren't all the same and vary in other characteristics that you listed. From the perspective of organics in particular different organic mediums have different Cation Exchange Capacities and qualities. e.g. I really prefer coir in my mixes and less perlite than most people would use. I like a certain variety of coir, but coir in general has more favorable pH characteristics than sphagnum, and also very good at remaining aerated while wet.
Otherwise, I don't really understand what you mean. For instance, the hydro store is definitely closer and more convenient for me than any nursery. And as I understand some carry products from the same brands or their distributors (i.e. SunLight Supply which distributes EVERYTHING). I don't see a problem with buying from a 'hydro' or more appropriately 'Indoor' grow shop. It is still supporting my local economy; better than buying stuff from online.
Honestly, can't be spending too much time preparing soils. It is still easier to take something like Ocean Forest and amend that a little bit or mix it with SAM#4 to a consistency I like and just add azomite and dolomite lime. OF usually isn't good for seedlings, though, and they don't need it. In a couple weeks they should be transplanted.
During this transplant lime should definitely be added to whatever you are using (Azomite has many trace elements and also raises pH). Cannabis needs a good amount of Ca (and Mg) to thrive, and you need it to be there long term even if you don't transplant during flower and use low ppm water. That alone is reason to add additional lime every transplant, and perhaps even something like CaMg+ to all water especially if your source is filtered, distilled or rain.