Break in Online GMAT

Yes, there is a break of 30 seconds before the commencement of the Verbal Section. A break is provided to read out the instructions before Verbal Section. The instructions don't contain any valuable information that you require, so you can use the break to gather yourself and relax your mind. But being a 30-second break, the break is too short to relax.
If you can finish Quants before time and have 5-10 minutes to spare, then don't submit your last question. If you will submit the last question, you will be directed to Verbal in no time. So, it's better not to submit the last Quant question before time, instead utilise those last 5-10 minutes as a break to relax. The Verbal directly after Quants feel tiring and drains your mind.

Yes. "Using" the time you have left on a section before confirming your last answer can be helpful.