Is a person considered an infidel if they abandon some prayers out of ignorance, while believing that whoever abandons prayer out of laziness is an infidel, and they used to pray one obligatory prayer every few days, thinking that this does not make them an absolute abandoner of prayer?

If you have repented from abandoning prayer, then praise be to Allah. The predominant view is that abandoning prayer does not constitute disbelief that removes one from the fold of Islam, unless it is abandoned completely. However, one who prays sometimes and abandons it sometimes does not fall into disbelief by that.