Uhm, regarding the copy function, after the first paste, it doesn't check again if you have the available nodes (will use whatever you have in stock, though), leading to a duplication glitch.
Belt mode can't be canceled with right click/ESC button, nor selection mode. Have to click on another module to exit the mode. Using the E key as toggle would also be a good idea.
On any query, the older info is shown first, then updated, when the order should be the other way around.
When clicking outside the building area, open space, resource nodes or a belt, the info window is briefly shown then closed. Belts and modules use different info windows, opening and closing in pairs.
The info window once opened is fixed to the screen and not on the module position, so when moving the camera, it doesn't update it's position.
What I meant with flipping, is like using the mirror image (is like a rotation, but using a different plane, in 3D). For example: on an horizontal flip, suppose the selection area is between x1 and x2, given a module on x, the flipped position is x2 - (x - x1). Use the analog with Y coords for the vertical flip (the actual names may be inverted).
As for the blueprints, just save the data of the copy function (module type, rotation, relative position to the start of the selection). Since you now store the last copy used, how about adding the paste function?
I recommend looking at Factorio for the blueprint and flipping (It's over engineered though).