Randy is right, 1 or 2 bad ones will mess everything up. I’m not sure about your other questions. It’s pretty warm to even need glow plugs but I still use mine as well.
I had a start issue, changed batteries, glow plugs, relays....I did have bad glow plugs (5 of them) and that helped a whole lot but also ended up being my starter not spinning fast enough. I hear we can upgrade starters now from newer fords to really put down some turning power but that’s a different subject.