there is a 5" round plate in white

Here is a link that might be useful: round plate

To avoid the makeshift look, the electrician on one reno installed a smoke detector at that location.

When you cut the holes for the pot lights, there's a decent chance that you will be able to see and grab the feed wire from the switch to the existing junction box. If so and it's just a switch leg and no other wires run through the junction box (usually the case), you can use it to feed one of the pot lights directly and then run a wire from that to the other light. This would enable you to remove the junction box and patch the ceiling.