Backup is different from the entire Iranian military. The Americans wanted to pull off a similar plan for saving the US diplomats. It didn't end well for them though, but it failed because of a sand storm, not because we stopped them.
Dude, 5 million illegal Afghans have crossed the border in 2 years. Do you think Israel can't send a few thousand commandos inside Iran in plain clothes? This is probably why Israel claims they need time to prepare for this plan. Obviously, they don't intend to heliboard them to Isfahan or Qom.
They don't even have to enter together at once. They can enter from different borders (and we have plenty of border with neighboring countries, most of them are extremely insecure and unguarded and we know for sure that the US and Israel are present there, particularly in Erbil, Republic of Azerbaijan, the UAE, Qatar, even Afghanistan and Turkmenistan).
They assembled a quadcopter that crashed into a military facility near Natanz a few years ago. They assassinated Ismail Haiyeh in the middle of Tehran. They assassinated our top nuclear expert with a satellite-controlled machine gun in Damavand. They killed and injured 6 of our nuclear scientists in the middle of Tehran with magnet bombs. So, it's not like Mossad has no presence in Iran and they haven't done similar things before.
It's still a stupid plan though.