I don't know the rules, or if there are any but, for sure, don't snug it up in the corner like in the first picture.

I think you're on the right track by propping it up & standing back to look at it. What works best will likely be a compromise between how it looks and keeping it accessible.

if flush, can you fully open the door?

no rules. do what works for you and looks best

I'd align the cabinet top with the top of the window frame and center the cabinet between the wall and the door frame.

-- amateur

There are no "rules" for cabinets. Rules are for electrical and gas.