How to decide if you're ready for the exam.

The official diagnostic scores are the ones to trust -- those tests are essentially identical to the real thing. So it sounds like you're around a 710 right now. There's variance on any test, because sometimes you have to guess, and some days you're luckier than others, so assuming you'll perform normally under test conditions, you can expect to score in the 680-740 range most of the time.

I don't know what score you need, but if you'll be happy with a 650, you're not likely to need more than one attempt, unless you're concerned you might be affected by test day stress or something like that. But in general, it's always best to leave yourself time to take the test again, just in case you have a very bad day, or there are technical issues. And if you need a 700+, you'll raise your odds a lot if you can take the test twice instead of once. So your dad's suggestion seems like a good one.

As for how to prepare, it's really hard to improve on Verbal scores as strong as yours, and if you were to do anything for Verbal, you'd want to practice exclusively official questions, trying to learn a few things from questions you find challenging. Your Quant scores appear to be more variable (based on limited data!) but that would make sense if you're prone to careless errors -- getting easier questions wrong hurts a lot on the GMAT, and if you sometimes make careless errors on easy questions, sometimes not, your scores will fluctuate more than normal. Figuring out how to minimize the number of careless errors you make should be a priority. Unfortunately it's impossible to give good advice about just how to do that, without knowing a lot about how you're approaching GMAT math, and why you're making careless mistakes. Sometimes test takers need to develop better habits (sometimes that just means writing legibly), sometimes test takers need to slow down and either reread questions or double-check their work in those situations where they know they're liable to make a mistake, sometimes test takers need to learn better methods so they can save time on some questions to devote to checking their work on others, and sometimes test takers are using awkward methods that require more steps than necessary, which increases the number of opportunities to make a mistake. If you can work out when and why you're making careless errors, you might be able to work out how to notice and fix them. Good luck!