Look how Iran reverse engineered US dome to make that brilliant stealth fighter plane and launched two different Monkeys into space

I really don't think the fact Iron Dome is on display at the Aero India expo is something to boast about as its just a display piece there. If it actually leads to exports then we got something to show off about but just featuring in the expo means nothing considering most arms companies display their products at these places.

What is to prevent it being bought by a country unfriendly to Israel?

Who buys Israeli weapons is vetted before any sales are agreed to. If Saudi Arabia wanted to buy Iron Dome Israel would refuse to sell. If India wanted to say we would love to do business with you. Russia wanted to buy Israeli UAVS Israel agreed to sell some of its older models but not the brand new combat UAV which they also wanted. Germany & France want the new UAV and sales have been agreed. The country we have to worry about is America. America which part builds and part funds the Arrow is talking about the possibility of selling Arrow to countries which Israel doesn't have good relations with.

within the US/Israel agreements about various missiles that a clause somewhere states that third party sales be mutually agreed upon. I assume this because Israel is not a vassal state.