With Qimage, no matter what I do and I have tried so many things, it likes to be smart and changes things for me in a very strange and illogical way:
No! Not illogical, if you specify a size, that's what you get, or if too big for the page, Qimage warns you - unlike other programs that kid you into thinking you are getting the size specified.
it tries to resize my images to fit INSIDE this default print margin of the parent sheet.
Of course, the printable area is controlled by the DRIVER and Qimage works within that. The printable area is reported, from the driver, above the page preview.
So I go into the Qimage settings and adjust margins, and make it so that the parent sheet size print margin is 0 for all sides
This will not work as it is controlled by the driver. Some drivers when set to borderless do allow small adjustments to the margins to correct transport errors etc.
Why can't it be more like photoshop
Because, Qimage is a "proper" PRINTING program and works with the driver and gives you
accurate data.
Is there a way around it?
You have already said that you trim the 4x6 prints, so specify a custom size a little smaller than 4x6; make the white border smaller.
or use a different size sheet; use 4x6 paper & set to borderless.
Sorry to be a bit critical but I think you need to re-think what you are doing and work within the real driver limits if you want to get Qimage print quality.
Terry