The thing I am not totally sure about (I cannot test it at the moment) is the question about the second line.
This is how it should work in theory:
Not specifying a mod condition should basically mean the same as [nomod] but i included it nonetheless
If you press the macro with a modifier (I used alt here) you create a healthstone
Otherwise you would simply use a Healthstone and therefore set the combined CD for Healthstone and Coastal Healing Potion except
If you have pressed no modifier and you have no Healthstone the macro will execute the first line (which does nothing) and then execute the second line. Since the CD is not set yet due to no consumption of a Healthstone the Coastal Healing Potion is used instead.
Personally, I know that this works since I use it:
/cast Healthstone
/cast Coastal Healing Potion
As I said, I cannot guarantee that it works but maybe you can give it a try.