Thanks for your lengthy description. It was very helpful in assessing in what you may be experiencing and understanding the bug on our side.
It sounds like... we did not keep your audio mix settings correctly in our project file so when you re-rendered your video in post- it mixed in another internal mic on an iPhone that picked up the audio also. This would create the echo.
A quick fix should be going back into the project in Cinamaker- hitting the 'Overwrite' button in the post editor, Select on 'Audio' overwrite then switching to the audio mix tab and resetting it correctly to only mix in your mic. Then go move the slider all the way to the beginning- hit record- let it run through with the new audio mix. Then export it again- and your audio mix should be completed...
Then- it should be easy enough to your new video from CInamaker into LumaFusion and render out your final video.
Hope this is helpful- please let us know.
Thanks again for the alert about the bug- we will investigate and address immediately.