Hungary is a NATO member, so it is politicaly impossible to sell those MiGs to Russia, not to say Iran. Only option would be, if they sell them to Serbia or maybe to Macedonia, Montenegro or to Poland for spare parts.
Yes they can sell it to Russia. And if not they still need russian agreement for selling them. Orban is a friend of russia he wouldn't care much about USA pressure.
Russia can then sell them to Iran. Both are under sanctions so they don't really care about western opinion. Moreover defensive weapons are not forbiden and mig-29A is more an interceptor than a multirole fighter like sukhois.