Do you really need to poll every 15 seconds? Rain sensor status doesn’t change that frequently — I think polling once every 5 minutes is sufficient.
Pulse rate is a constant — this depends on the spec of your flow sensor. The flow count is a number that’s increasing, not pulse rate. It’s like saying the speed of water is constant, but the volume of water flowing through the pipe is increasing.
Sure, values are resetted when rebooting.