For the difference between European and American butter, I am French in France, so I use European butter, and bake a lot with King Arthur and American recipes and I simply withdraw 10 to 15 % to the total butter weight (I tend to not like fat) and have consistent great results with this tip.
Kick tip for French bakers when creaming butter and sugar the Président beurre gastronomique is the best : it is really softer at cold temperatures than any other brand, so you can have great creaming results withe 15°C butter and no risk of over creaming because of to hot butter. Specially in summer.
On the other hand Paysan Breton for example is too hard, even if I prefer it for tartines (thee salted version of course :-)

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