Have you asked whether they provide in-house alterations at M&S? I know they do at Zara.
Sleeves shortening is quite common, you can try Za Altera. I'm sure there's an alteration shop in KLCC but pricier.

Adding buttons? What buttons are you talking about?

Making working cuffs? Too much work (I'm no expert though), might cost a bomb. Why bother?

I think they don't actually do it in the store itself. I assume they send it to someone for simple alterations like length.
I asked the SA at Pavilion and he told me I could bring my garment to any Zara outlet. So, yeah.