If a script tries to move a movie strip to a channel that is full, does the script abort with an error?
Is there a flag I could use, to increment the strips channel so it moves the strip to the next free channel?
How could I do this?
If a script tries to move a movie strip to a channel that is already full, it automatically moves to the next empty channel.
You can aways use the "G" shortcut to grab along the Y axis in most editors, including VSE.
Just press G and then Y.