Accuracy of GMAT Official Practice Tests

The official practice tests are identical to the real test -- same scoring algorithm, same kinds of questions -- so unless you were familiar with some of your test questions in advance, your score should be perfectly accurate, once you account for variance. If you take another test, it wouldn't be surprising to see a score twenty points higher or lower than this score, because luck plays some role in scoring, but if you're aiming for a 710+, you could honestly go into a test center tomorrow and be pretty confident of getting that score.

If you were planning on doing a bit more Quant prep, I'd do that first, then take another official diagnostic. The average of your two official scores will give you the most accurate assessment of your level, and should give you a good prediction of what to expect on a real test. It sounds like you're doing great -- good luck!

edit - and I forgot to answer your second question. There is not, as far as I'm aware, any overlap between the OG/QR/OG Advanced questions and the questions in the diagnostic tests. If there is any overlap at all, they made a mistake (a mistake GMAC has made before, which is why I'm qualifying my answer a bit, but I've never noticed any overlap between the recent books and the recent tests).