Any way to universally change margins/gutters in Acrobat?

Good day, I would love to be able to assist you but to be honest, to date  there is no tool that will  increase your mirrored  margins, to my knowledge. 700 pages equates to 350 double sided print production. What you need to do is export the doc to MS word then copy and paste each chapter seperately into the MS app notepad++  This process will convert the text to a workable word file. Then you copy and paste that text back into an unformatted narrow equal sided unformatted blank document, each chapter  at a time. just keep going with this process until  the entire document, chapter by chapter has been through the cleaning app...no need to worry about page breaks, as long as you retain the chapter numbers. Once completed, you have to save your document, then and work on word spacing but it is doable. You need to dedicate a day or two to this task, but once completed you have a workable word document. Sometimes you need to recopy the whole file back into the app to get it to a tidy document, and tou can chang eth font style in word if you wish. PDF is a snap-shot and of font that are like the bricks in a building, so think about just being able to slide a few rows od brich around it create a window.🤣🤣🤣 MS word is like wet cement, and you can move and slide it according to page size. Notebook ++ is a free MS app. Good luck 

Yes, I'm there. Thank you both for pointing it out and it gets me closer to a solution, but of course cropping a page involves removing part of the page and then it is unfit for publishing. Is there any way to add back some white space and "mount" the cropped text into the correct position?

If you have Acrobat Professional, then look at the "Crop Pages" tool, which is in the "Edit PDF" toolset. 

You can use the crop tool.