Can you balance osmo mobile with iphone x leaving the lightning plug to connect an external microphone? Someone has tried and what can do if it does not work?Thank

That is hard to say, the size of iPhone X is almost the same as iPhone 7 plus.
For iPhone 7 plus, once it is balanced well, there would be no enough space for external mic.
Sorry that iPhone X hasn't been fully tested yet, hope other users who own the iPhone X could give you some advice.

I can't confirm this as I am still on iPhone SE, but you would need a very small lightning connector for this with a 90 degrees angle since the port is centered.
As soon as I find a person with a X, I will give it a try
You might be better off with a wireless version via Bluetooth.