Hi Octo,
You have a bunch of questions here… I'll just touch on a couple that come to mind.
IMHO, no single ISO or EI is going to work for all conditions. A bright sunny day might require more exposure for the shadow detail because of the reduced development times. And a flat, gray day might be OK at the box speed of 400 ISO because of slightly increased development times.
You have to do what works for you. Everyone has different equipment, and very different ideas about what a good print looks like.
I first look at shadow detail on the negative. If the negative has too little shadow detail, you're going to have to give more exposure.
Now, you could have plenty of shadow detail, but if you have over-developed, you'll end up crushing that shadow detail because you have to blast the print with light to get the highlights to show. Too little highlight density and your prints will look flat.
So, it is a delicate dance here. Enough shadow detail, in combination with highlight density on the negative that prints (or scans) easily without crushing the shadows.
Some people have great success with shooting gray cards and measuring densities (been there, done that). But for me, going out in the backyard and shooting a few quick test rolls under different conditions has been the best learning experience.
Good luck.