Nobody – I don’t need an excuse to wear a slit because I don’t hold that its wrong to wear one. If someone is trying to excuse it, it means they think its wrong and are trying to convince themselves.
As for wearing a shorter skirt if you need a slit – in the winter, I prefer to wear long skirts because its cold. I generally wear tights underneath, but frequently need a slit in the skirt to be able to walk comfortably. Not a problem for me, because my Rabbi doesnt hold its assur. If you hold its assur, please do not wear a slit. You would be violating your psak.