Well let’s say Cruz does win the nomination. What will the Dims do? Will they make it an issue? Will they sue as Trump says? Would it go to the Supreme Court? How would the Court rule? And what would happen if he were found to be ineligible? How would a new nominee be selected?
If Cruz is the nominee, that "finding" will only be whatever traction it gets in the press. It will affect some voters, and there is only downside risk to Cruz on that (meaning the most votes come if he is clearly qualified, and gets fewer votes if there is a cloud).
If Cruz becomes president-elect, Congress can find him unqualified. I doubt it would, but it could, meaning it has the constitutional authority. Either way Congress goes, a lawsuit could be taken to SCOTUS between the time Congress finds Cruz qualified, and swearing in. SCOTUS could refuse to adjudicate the question (meaning there is no check on Congress seating an unqualified president-elect), or take it up.
There is no new nominee. The unqualified person finds something else to do, and the law seats the veep-elect. If the veep-elect isn't qualified either see 3 USC for next in line.
If Cruz wins the nomination then Trump can sue but it will be dismissed on lack of standing. The only Democrat who would have standing do sue is party nominee. And they will have standing only after they lose the election. So regardless, the suit cannot take place until Cruz has been elected president. And if, for the sake of argument, he is found unqualified then the vice-president would become president. But if there is a suit and if it isn't thrown out of court then the almost certain verdict would be Cruz is eligible.
The GOPe electors would then just shove Jeb! down our throats.
Just having the question unanswered and in courts during the campaign against Hellary would suck the wind out of the campaign.
Both the Rinos and the Dems will go after him.