1) The fix you did is likely the fix. It's working now, hopefully stays that way and isn't something electrical. You mentioned seeing a YouTube video of a mechanic accessing the trunk through the back seat - was it a sort of nasally guy with a flat blade screwdriver? If so I know that guy.
2) Most Malibu's are the non-hd camera since the screens are too small to benefit from 720p or 1080p. The Premier maybe newer down a step 2LT have the "HD" camera which is mostly useless on a 7 or 8" screen. I don't believe the lense can be replaced as an official process but I'm sure some hobby techy person could take it apart and rig something. Since your trunk button is now working, I'd start by disconnecting the camera and check the wiring. If it is just cloudy, probably easiest to just try a replacement. You could also try some rubbing alcohol on a microfiber cloth or lense wipe on the lense prior to replacement.
These are the only two issues with the car? No other odd electrical things happening or performance stuff?