1) 15 seconds divided by three is 5 ticks of 30, 5 times 30 is 150. So if it runs the full duration (give or take) you get 150 potency.

2) It's very situational. Generally, you want to clip ("overlap," as you called it) as close to it running out as you can, but within the last 3 seconds is usually close enough. At that point, you're deciding between a tick lost to overlap and a tick lost to not refreshing in time.

3) Yep. Otherwise, BRDs would never get River of Blood procs.

4) Whatever buffs (and certain debuffs) are up when the dot is applied will affect the entire duration.

5) It's a few extra dps, but most groups will only use it for progression.