Use a better chocolate for starters, use a higher cocoa solids content and it will melt thinner and be easier spread, I'd suggest something around 50% cocoa solids in the Moser Rother from Aldi or the Lidl ones, can't remember their brand. You don't want a really sweet chocolate as the powder is already sweetened.
I would just store at room temp when made in something airtight, should last a couple of weeks anyway.
