POTS sufferer chiming in here. I have an S4 (which doesn’t answer your questions). However, I’ll say that HR is accurate. I compare it whenever I’m at the doc’s office. The native HR widget doesn’t update often enough. I would suggest using TachyMon, an app built for people with POTS that works on the watch and can send vibrating alerts when your HR passes thresholds that you set. It drains battery quite quickly though, so you may need to charge up more often. My HR also spikes in the shower, and I’ve fainted once in the shower. Purchasing a shower stool was a lifesaver for me.

Check your local TJMaxx or Marshalls. You can find Apple OEM sport band and sport loops for $13. If you don’t like the color options, you can dye the sport loops! This is the route I went since I wanted a black sport loop, which Apple doesn’t make, and wanted OEM-quality lugs.