Hi,
As you have found, iPad iMovie adds transitions by default The only way that I can see to override the default is to select each transition individually and choose "None" from the selection menu that appears at the bottom of the screen. I could find no way to turn off the default setting, or to set the whole project to "None", or to remove the transitions as a group.
The workflow would be to add your clips, remove the transitions individually, and then add your audio.
A workaround would be to add your clips without transitions to a Mac iMovie project, share it out as a movie file on the desktop, and then Air Drop it to your iPad Photos app for import into your iPad iMovie project. A bit awkward, I know. Maybe you can come up with an easier way.
Another way would be to import your clips into QuickTimePlayer on your Mac, and export it out from there instead of using Mac iMovie.
-- Rich