Does your machine have the Business Ready (BR) or Production Ready (PR) finisher? The BR finisher has a stapling limit of 50 pages while the PR finisher has a limit of 100. I'm in agreement with you about the math involved here during printing because I can't figure out how your combination of pages adds up to over 50, even if I count the half-z folds as 3 pages each due to the folded thickness. Here's what I recommend doing next.
1. Try to build this job using the copy function and see if you still hit a stapling limit. If the machine will do the job as a walkup copy then that justifies starting a log with Xerox support in regards to this. If it doesn't then it sounds like you're configuration is working as designed.
2. Try to use the V3 Global PCL6 driver. What you've said doesn't sound like a driver limitation, but the V4 does lack in some ways compared to the V3 driver. If the machine was sold with both a Fiery and a PS enablement kit than use a PS driver instead.
3. The EX-i Fiery should include a Fiery Compose license. Try building the job in Compose before printing to see if you get a different result. I don't expect that you will, but it's included so it doesn't hurt to try.
Please update this post to let us know what the ultimate conclusion is.