All those ideas are great - I don't really like the one about opening the PDF in illustrator - that's opening a can of worms. There could be font issues and text moves or slides behind other things, clipping masks etc. can go awry.
I'd avoid opening in Illustrator unless an absolute last resort.
The Touch Up in the Acrobat Pro is the best bet I think and will probably do the trick.
Other than that masking it in InDesign is a good bet too.
Thoroughly agree that getting the Source Files is best!
If you have can't get the source files then you should recreate the document properly when you get time so the next round of edits is easier.