I might hook this up a bit differently...
I would take the hot wire (power input) into the common of relay 1. Then take the NO contact from relay 1 into the common of relay 2. Relay 2 NO contact connected to the fan's high speed input, relay 2 NC contact connected to low speed input. Just wire nuts connecting the neutrals and another for the ground.
This would allow relay 1 to power the fan on or off, and relay 2 would be for speed selection. The advantage is that there is no way for the high and low speed inputs to be energized simultaneously.
Also, most electrical equipment is rated for use at 104 degrees. This works out to 40 C. And is the "standard" for almost everything. Higher than that and you should start derating the wiring, relays, etc. As you noted, you might also see some impact on service life of the electronics.
