Why can't you use push connectors to extend a short wire when they are used like that when installing LED pot lights in parallel?
Use a NM splice kit
These sort of rework applications are precisely why Tyco and others make special NM splice kits designed to provide a reliable, secure, self-insulated connection between NM cables without the need for a junction box to enclose it. So, I would use one of those to whack an extra length of 14/2 NM onto the too-short wire (make sure to follow the instructions on the splice kit carefully!), and then proceed as normal.