Hmmm … not sure you need to go to all that trouble. Skuid Save has functionality to revert all changes on error.
Further, the Skuid IDs aren’t stored anywhere and are unique to each session. So you can refresh your page and the same records would have different Skuid IDs.
Skuid IDs are always entirely unique. At least this is my understanding and would be quite surprised otherwise. 
As for relating Line Items, you can do this prior to save using the temp Skuid ID of the Invoice. The one thing I’m not certain about is the APEX bit. Fairly certain that any error in APEX would cause the save to fail and everything would be fine.
Hope this helps. Maybe I missed why you felt you need to use temp skuid temp IDs in Apex. Lemeno
