Pretty much all of the above
It is hard to say. I mean, it is half the number of segments but an index that gets written to constantly is going to have dozens of segments and that performs just fine for most things.
Slower, I think.
I'd do one, but only after I'm super sure I'll never write to the index again. I don't expect you'll see a huge difference between one shard and two but it might be worth testing if you are particularly concerned about performance.