Is it kufr for someone who prays, yet has missed some prayers (for a year or a short period), if he resolves not to make them up, despite the existence of another fatwa stating that making them up is optional and there is no proof for it?

What is stated in the fatwa (definition) represents the opinions of the scholars, and the mentioned discourse was transmitted from Abu al-Hasan (definition) al-Ma'ribi, and it is an ijtihad (definition) (independent reasoning) that may be accepted or debated. The issue is a matter of ijtihad and not a place for condemnation. We incline towards the view of the majority that the one who abandons prayer does not disbelieve with a disbelief that expels him from the fold of Islam. As for making up prayers deliberately missed, it is a point of contention among scholars, and it is a matter of ijtihad.