Yes. The idle speed is roughly 10 times the voltage at which the fan will come on. For example, if you have your idle speed set to 40 percent, the fan will come on at 4 V; if you have it at 80 the fan will come on at 8 V.
No. If there is a low or high temperature alarm the, alarm contacts will change state but the variable output will continue to operate according to the programmed settings.
The output rating is 2k Ω of load. Each device or wire attached to the output adds to the load. The total number of VFDs it can control depends on the VFDs and well as the length of wiring connecting them.