I agree that it looks like a blouse and a jumper*. The jumper can have an ordinary back zipper, but it is pleasantly short because the jumper stops just about the shoulder blades in back. The bow will tie over the zipper, hiding most of it. I may have a jumper pattern, I'll go look in a bit.
The blouse isn't necessarily fitted, it's just tucked in to a fitted jumper. Make sure the tail of the blouse is long enough to hike up your skirt and give it a tug after a lot of movement to keep tidy. The longer the shirt tail, the less upward creep you will have.
I think your sleeve plan is a good one, mostly. I would not carry the tight fit sleeve all the way to the shoulder, it will likely restrict your movement severely. I'd attach it at the top of the cuff (bottom of the puffed section). You might look through Vogue patterns for a close fit sleeve to borrow from rather than trying to work it out from scratch.
You are on the right track!
* a skirt with a bib front attached. or a dress designed to have a blouse underneath, think very small childrens school uniforms.