I just failed in the custom fence interference approach. The base game just crashed when a lot with wall1 walls at the edges is loading...yike!
What I did in this approach:
I used a custom fence and alter its guid ID into wall1's to override the default wall1!
Then, I built the custom fence up to the lot's edges and save the lot and quit the game.
Then, I remove the overriding custom fence before I entered the game again.
The game just crashed again and again when the lot loads.
So, this approach failed to get the lot stable with wall1 walls at the edges of a lot.
On another approach like either using the lot-expander or the lot replacement approach, an overlapping illusion of combined lots should workout. But, keep in mind, these lots are modded lots, and I'm unsure how stable and safe to save a back-up for these modded lots. But, hey, without testing these out, we'll never know the result! :D