I am replacing my one-speed 230V 4.5HP pump with a new one but the supplier mistakenly sent me a two-speed pump. Can i simply connect the power to the High speed on the pump and disregard the low speed altogether?

Yes you can

I hooked up the pump just on the high speed and it works really well....for about 1 minute and then I get a "hot" message on the control pad...the temp goes up and then triggers "HL". This all happens within about 20 seconds. Could it be some kind of heat being generated in the Spapack from hooking the two speed up as a single? If I disconnect the pump or run it for only 10 seconds or so...no problem.

The HL is caused by your heater, not your pump. You aren't getting flow through the heater assembly.