Is offering dinner before Isha prayer—given there is ample time for the prayer and no necessity to delay it—considered a form of turning away from and neglecting the matter of prayer, and preferring other things over it? And does this reach the point of disbelief or sin?

Congregational prayer is an individual obligation (fard 'ayn), and performing it in the mosque is more meritorious and preferable. However, if it is performed congregationally elsewhere, it is valid, but the virtue of the mosque is missed. Delaying prayer beyond its prescribed time is a grave sin, greater than adultery, theft, drinking alcohol, and killing a soul. Some scholars have even gone as far as to consider one who intentionally abandons it to be a disbeliever. Among the excuses that permit missing congregational prayer is the presence of food when one strongly desires it. Presenting dinner involves either delaying the prayer beyond its time, which is among the major sins, or abandoning the congregation. If you have a need for food, you have a concession; otherwise, you are blameworthy. If the congregational prayer in the mosque has already been missed and you pray congregationally with your family within the time, that is permissible, but you will have missed the virtue of the mosque.