If by "belong" you mean custody, then yes. If you mean rightful property, then no: If you own something and somebody forcibly takes it away, it is still rightfully yours.
But of course rights are of not much use unless they are enforced.
No, the rights of the new owners supercede the rights of the displaced. The displaced may never acknowledge the right of the new owners to possess the land, but ubless they have allies that concur and would seek to challenge the new owners then the displaced will only remain bitter and displaced.
PS: There is no such thing as international law. Such "laws" are merely treaties among states, who may choose to abide by those laws or not as they wish.
Thus, the problem of the Palestinians. Their claim to ownership is entirely dependent upon the benevolent moods of a semi-impotent organization (UN). Since they are vastly overpowered by the Israeli's a normal course of action would be for them to surrender entirely and rebuild their society within a new pacified government structure.
As long as the UN and the US keep Israel on a leash favoring the Palestinians then there will not be peace. The Palestinians must be denied all hope of eliminating Israel or having their way with Israel.