While the mother is alive, can the Gaya shraddham be performed for the deceased father after the first death anniversary? Can the younger son perform Gaya shraddham before the elder son due to travel conveniences?

It is not wrong to do Gaya shraddham after the first death anniversary of the father, but one has to go again to offer pindam at Gaya when the mother attains Paramapadam.
Looks like it is not wrong if the younger son performs rites at Gaya before the elder son does.