Must I repeat the 'Isha prayer after having read a fatwa that invalidates it, while there are other fatwas that affirm its validity?

You did not mention in the question what happened to you during the prayer. In any case, the nullifiers of prayer have been clarified in Fatwa No. 32938. You should adopt whichever of them seems more convincing to you and puts your heart at ease. If you decide to adopt the first fatwa, you must repeat the prayer.