We have outsourced a lot of graphics work externally; they use Invision and Sketch on Mac. Now that the design work is drawing to a close, I need to do the clean-up. I’d prefer to not have to re-create the wheel and start over in learning a new product (from what I have read, Axure is better)… has anyone successfully imported a large Invision file?
InVision to Axure… That’s an interesting concept. “Interesting” in the way a vegan, gluten-free lasagna is “interesting.” 
Technically, you should be able to do this with the inline frame widget, but it appears InVision blocks this:
https://forum.axure.com/t/embedding-an-invision-mockup/29690/4?u=mbc66
I guess it depends on what kind of clean up work you need to do. My guess is what you really need is Sketch to Axure, which exists:
https://forum.axure.com/t/check-out-our-axure-sketch-plugin-for-bringing-sketch-assets-into-axure-rp/50269?u=mbc66
I say this because InVision is basically a presentation tool with some nice collaboration and dev support features, but the real design work is done in Sketch, Photoshop, or Illustrator (unless maybe the beta Studio app is being used, but that is Mac only and definitely still beta.) It’s just not a good prototyping tool in my opinion. You can make better prototypes in PowerPoint or Keynote.
Now all that being said, you’re probably already pushing a rock up a big hill if your broader team has already been using InVision and you need them to consume your “cleaned up” designs. Switching presentation/delivery tools midstream can cause lots of problems. So, you may discover you also need a solution for “Axure to Invision” after you’ve cleaned up things.That’s something I’d be interested in trying out.