1.Well you can use photoshop if you don't find coloring on Clip Studio Paint/Manga Studio comfortable, but to be honest stick with Clip Studio Paint/Manga Studio since it's also a decent coloring program.
2.I made my lineart settings look like vector-ish, and no the resolution and the size of the canvas doesn't affect the lineart.
I also adjusted my pen pressure like this, since it's more easier for me to ink with that settings.
(btw, the resolution is used for printing quality, 600-1200 resolution or dpi are used for printing purposes, tho on the program itself the resolution affects the text size and the lower the resolution is, the smaller the text gets, and for importing images inside the canvas, the higher the resolution is, the smaller the image you imported inside the canvas.
The text "test" is size 9 on different resolutions.
(Resolution: 75)
(Resolution: 300)
(Resolution: 600)
3.I use the "the lines looks the same" because it matches pretty well with my style, but almost all of the users use the pen pressure, so yeah, pen pressure.
4.Sketching first, and a -wild imagination-
5.I'm not the guy for this question because sometimes i pick the colors semi-randomly, but i guess experiment yourself with coloring and stuff.
When i color my cartoon style, i just slap on some colors, but when i color my anime style, i use the airbrush tool and the gradient tool to make it look "shiny", as for the skin, i use HSV, and i usually just remove 20 for the V and adding 5-10 for the S, idk if im doing it right, im still learning how to properly color and to shade properly.
(I only colored this drawing, i didn't made it and it's not my character) <--just putting this off, i dont want to make my friend angry.