You could go up a size for the sleeves or just extend the increases to make the sleeve wider but if you widen the sleeve, you'll need to check that it will fit into the armhole opening. Of course, you can always gather the top of the sleeve cap a bit to ease it into the opening. It might be best to finish the front and back and then join them at the shoulder before re-doing the sleeve. That will give you an idea of the size at the armhole. The fabric is supposed to be quite stretchy according to the Rav notes so perhaps you won't need to increase very much?
I wouldn't worry too much that the shaping isn't exactly the same between sleeve and sweater body. You will seam along the edges even if they don't precisely align.