Glad I could help!
Yep, always melt the copha and soften the butter / margarine!! Sometimes when you melt Copha, you will get a small amount of clear liquid in the bowl, just smush the white and the clear together to get the opaque-ness again. Don't melt the Copha so far that all of it goes clear though, it has to remain creamy and opaque. If you do go a bit far with the melting and you end up with a bowl of clear liquid, just set it aside, stirring and smushing with the fork regularly so it cools evenly and starts to thicken and get the opaque-ness back again. Definitely don't use it when it's clear as it won't behave and you'll just end up with frustration and a bowl of weird icing haha. Hope I explained that alright?? It's just a really important step :)
I only ever use the half margarine / half Copha recipe and I've never had any issues with it being affected by heat / humidity and it smoothes and crusts really well.
And I don't use expensive brands either! Homebrand Canola Margarine Spread and Homebrand Icing Mixture from Woolworths supermarkets work great and taste exactly the same as the expensive ones!
Using margarine also produces a more white coloured icing. If you use real butter, then the icing tends to be more yellow.