EY will be no help on this.

You officially do not need a visa for India transferring to KB to Paro. At the airport when you go to the transit desk, an official will walk you to arrivals, take your bags to recheck at the KB desk, and walk you back through immigration to the international transit area. But only if there is not lengthy layover involved.

But, as EY might deny you boarding without an India visa (by mistake), if you want to be 100% safe, it is better to either get the e-visa now offered for $60 on-line to many nationalities, or to apply for a transit visa. Better anyway because there are sometimes lengthy delays.

When checking in at KB ask for the upgrade cost-often very low and you'll get a nicer view of the mountains. Sit on the left of the plane from DEL.