Those scores (680 and 730) actually are within the standard deviation of these kinds of tests. Standardized tests are unfortunately not as precise as we would like to think they are.
The standard deviation on our own tests is approx. 50 points. The standard deviation on the real test is approx. 30 points. (In other words, roughly speaking, scores within those ranges are not statistically different / someone with the same ability can expect to score in that range, not just the exact same score every time.)
So you do already have a clear picture of your current scoring range: it is in the high 600s / low 700s. That's about as precise as you're going to get with the GMAT.
If your MGMAT scores are at the lower end of your range (and GMATPrep at the higher end), then you may have some timing or mental stamina issues. In that case, you'd want to focus on beefing up those areas before you take the real test.
It sounds like you are already moving in the right direction on timing (letting go of certain problems), though you may have a little further to go. Most people should do this on 4-7 problems in each section. This is true even at top scoring levels. In January, a student of mine scored a 51 on quant - the top quant score - and she skipped* 4 questions in the quant section.
*skipped = guessed almost immediately and moved on (since you can't actually skip!)
The articles that I linked in an earlier post will also help with this.
For mental stamina, first, recognize that you have a finite amount. As you make decisions about when to keep going vs. when to guess and move on, don't just think about this problem. Also think about whether there are better places to use the time and mental energy that you're proposing to use on this one.
Also, on a more granular level, this article on decision fatigue can help:
http://www.manhattangmat.com/blog/index ... you-crazy/If you'd like to get more detailed timing and content advice, use the below to analyze your most recent MGMAT CAT (this should take you a minimum of 1 hour):
https://www.manhattangmat.com/blog/inde ... ts-part-1/Figure out your strengths and weaknesses as well as what you think you should do based on that analysis. Then come back here and tell us; we'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)