Those gerbera daisies are wonderful, magnificent, and glorious! You have a real talent.
Just to reiterate, all of the mousseline flavorings/additions are added after the buttercream is finished and the butter is completely emulsified and creamy.
Ideally your meringue and butter should be close in temp, 65-70F, and it’s OK to put the meringue, covered, in the fridge. If you were making a larger batch, it will take even longer to cool. It’s also OK to dump all the meringue into the butter at once and beat, just like Rose does in her video. Temperature differences usually affect the intermediate curdling stage the most, rather than the end product, which normally still comes together unless the meringue was so hot it melted the butter.
So it sounds like your buttercream was almost leaking liquid, is that right? I’m not sure exactly what caused it, but maybe we can figure it out.
A few questions:
-Was the buttercream completely emulsified and smooth before you added the white chocolate?
-Was the sugar syrup the right quantity and at the right temp (248-250F) when you poured it in?
-Do you think either warm meringue or warm white chocolate could have caused some of the butter to melt?