Yes to all of the above.We have a jump pack, not sure why I never thought of trying it, definitely will in the morning. I had a thought, the negative cable end, word is slipping my mind right now apologies, has always been kind of loose it’s tightened as much as it gets but I can still work it off with my hand. Could that be causing problems, I imagine it isn’t helping, I’ve been meaning to replace them anyways
You can tighten the negative by removing the bolt and then file the inside flats (red arrows) that are making contact prematurely
But again put the negative lead on the terminal not the post and do a cranking voltage test.
Record the voltage then do the same but this time on the post of the negative.
If the voltages are the same the contact of the terminal to the post is ok. If not that illustrates what I was saying about the wire with only one strand. Too much resistance under load.