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We've added option "keep fronts and backs together" that controls this. Check it off to get the "old" behavior.
With the latest version here https://pressnostress.com/2017/04/27/iw-210.html you can both edit trim box and shift crop marks and trim line.
Done by renaming the "Booklet" option in the latest version: https://pressnostress.com/2017/04/27/iw-210.html
Please check our latest beta version of Imposition Wizard: https://pressnostress.com/2017/04/27/iw-210.html - the feature is done there (you can also drag and drop PDF files into the application).
You can edit trim box with the latest IW: https://pressnostress.com/2017/04/27/iw-210.html - bleeds are not editable, as we automatically cut them in case of intersection.
The latest IW lets you both correct bleeds and shift crop marks: https://pressnostress.com/2017/04/27/iw-210.html