Hey Rescueme,
Leaving the struts inflated wont hurt your kite but the necks of the valves will stretch having the stopper in all the time. The struts will also slowly go down so you will have to pump them up hard next time anyways.
The valves can be reset by pouring really hot water over them or soaking them in hot water and allowing them to cool.
You've already mentioned the hot car thing .... bang

Good winds,