Hey HumptyDumpty,
Thanks for the debrief, and it's great to hear that our scoring holds up so well to GMATPrep!
As for your situation, a few thoughts:
1) There just isn't a better predictor available than the GMAT Prep tests, so if you're worrying about which tests to trust, trust that one.
2) Regardless of the scoring algorithm / question quality on that most recent test you took, there are plenty of factors that could have contributed to a lackluster performance - your stamina might have been sapped as you near the finish line, you may have been a little distracted having done so much work leading up to it, etc. And not to make excuses for you, but take that lesson as "When I'm on I'm mid 700s, but if I let my attention wander, if I don't go in well-rested and focused, etc., nothing is guaranteed". There's something to be learned from that most recent test. Try to replicate the conditions from those 730s on test day.
3) Maybe most importantly...YOU CANNOT FAKE A 730 on GMAT PREP! Unless you saw a bunch of repeat questions, that 730 isn't a fluke, and with a very-similar breakdown 730 on the Veritas test you have some validation that it most certainly wasn't a fluke. Be confident - no one scores 700+ on one of those tests by accident, so it's pretty clear that the
MGMAT test was the outlier, for whatever reason. You have 700+ potential this week, so be confident in that, and keep doing the things that led you to those back-to-back 730s recently and you should be successful on test day!