"NTRADES = 0 can mean not implemented or no trades"...yes it can mean both, which to me does not make much sense.
Anyway, then the script has to figure out if the comnmand is not supported or there are zero trades.
This is not very effictive nor clear I guess.
I already implemented this command and was wondering what the caller would expect here.

For not reseting values: this means that if the script user does not set a new value strange things can happen.
If SET_LIMIT is not set for every new order related command, then this limit price might stay for totally unrelated assets.
But Ok, if this is the desired behavior...