I followed Phoenix-D's advice and his solution works somewhat well, but I still must check for available artillery in range before clicking with the artillery remote; it will send in a MIRV if I click somewhere out of range with the artillery remote. There has to be at least one cannon with a shell in it within range of the target. It means I cannot click multiple times without sending in MIRV accidentally, as you quickly deplete the cannon magazines.
And checking available artillery is tricky: the count on the artillery remote icon is incremented with the count of available MIRV; you have the check the difference.
Example: If I have 60 MIRV in orbit and 3 available artillery to fire, the artillery count will show 63, but it's really only 3. If I click four times, the fourth one will be a MIRV.
But at least, the artillery can be used, with cautious clicking.