The ideal temp for both the cooled italian meringue and the butter is aroud 70-72F. Could be you're overcompensating for a hot room and the mixture is too cold. Or could be the hot room is having its way with your ingredients and they're too warm. Either way, try to get both bowls to 70F before mixing the butter and meringue together to minimize curdling and promote emulsion. Your butter proportion is lower than what I've seen elsewhere. For 75 grams of egg whites, I would use 185g - 227g of butter (that's about 2.5 large whites and 6.5 - 8 oz of butter). High-fat, euro-style butters work especially well in buttercreams, so maybe the combination of a low proportion of butter with a less rich brand produced your problems. If you're adding the butter to the meringue a spoonful at a time, it is normal for the mixture to thin out dramatically, then curdle a bit, then thicken up and emulsify beautifully as you add more butter. So maybe you're just not getting to the last stage, by not adding enough butter. If you still have any of that soupy mess, I'd be interested to hear if it helped to bring it to 70F and whip more butter into it on high speed.
