No need to raise the tank. As long as it's within 10 feet (which it is) and on the same level it''ll thermo just fine. What you're doing looks good so far. Make sure that there are no valves between the PRV's and the pipes or tanks (looks like you're good so far). If all valves are shut off, the PRV's must still work. I think you may plumb the PRV's together and out.I don't think code talk about two PRVs together. Again, no valves. I might plumb them seperatly, and do it with as few elbows and as short a run as you can. The idea is to make it an easy path to blow off.
The overheat... I wouldn't worry about it. From what I've read they are designed well and rarely give a problem. If it does, then cross that bridge. I'm not sure how you'd make a dump zone. If you made a thermo siphon one, it would be hot all the time, not just when it overheats. If you put a pump on it, you'd need an aquastat. Kinda complicated for something that'll probably never get used. Maybe someone else has better words for a dump zone.