Then add it to the parent.
You would pass to the parent then inherit for each repeat.
Parent
text LineKey Line Key
In the repeat
hidden RecKey Line Key ${LineKey}
Jim pointed this out above already
Regarding your second query - the field: parameter will not work with mode=edit and is not supported for questions in a repeat. If the APN is stored in the parent layer, one option would be to make the APN question in the repeat read-only and use a calculation to make it inherit the APN from the parent.