"I will go to Shanghai to see a friend."
Without 一个 the sentence is ambiguous in Chinese. It might mean ‘a friend’ or ‘friends’.
So it’s a more exact translation with 一个 but I honestly can’t answer about whether it’s absolutely necessary.
Yes 看 is also acceptable, it can be used the same way in english to when we say "I'm going to see a friend". 见 Can be translated as to see or to meet so you "can see" how they are interchangeable here.
I’m happy with the translation given.
However, I wrote: 我会去上海见面朋友 off the top of my head. I can see why their translation is better. Mine is vague about the number of friends, and i’m not entirely sure I’m using jianmian correctly. Anyone here know?
见面 is a thing you do with someone rather than using that someone as the object of the sentence, for example: 我会去上海跟朋友见面
Because 看见 doesn't mean seeing as in meeting (like here), but instead literally seeing like with your eyes. As other people have said, though, 看 is also okay.
I don't know why, but 会 in these sentences always makes the action seem involuntary. It's definitely not how I would speak in real life.