The way that I'll explain the concept, from a high level, would be that the dwelling unit would need to be disposed proximate to the sale of the land that is subjacent to the dwelling unit if the gain on the sale of the land is to be excluded under Sec 121. A demolition is a disposition for zero dollars of consideration, and I am comfortable with the position that if a dwelling unit is demolished or destroyed, and the subjacent land is sold thereafter within the time confines of Sec 121, the land qualifies for the Sec 121 exclusion as long as the prior dwelling unit would have met the ownership and use requirements under Sec 121 notwithstanding the demolition or destruction. There's really more to it than that, but it would take 10-15 minutes worth of talking to fully explain, which is a little too much for a forum post.
The sale or exchange of the dwelling unit requirement that you're reading in both the majority and the dissent of Gates is in effect an anti-abuse rule to prevent someone from engaging in behavior such as: buying land, putting a mobile home on it, then moving the mobile home to a new lot and selling the old lot every couple of years while utilizing the Sec 121 exclusion on each lot sold. In that situation, the dwelling unit is never disposed, and that is the essence of the dynamic you're trying to examine.
Now, with the situation at hand, you're talking about a lot more than just scrapping the mobile home, so the optimal answer given the facts and circumstances would depend on a conversation with a tax advisor who excels in this area. And I'd reiterate, that if you want to keep things simple, just sell it as-is for the lower price. That's less risk and higher probability that the property qualifies for the Sec 121 exclusion.
Again, it's my opinion that the majority in Gates is offbase. That they got it wrong. It happens, that's why we have appeals. The dissent is really the best part of that case. But even then, Gates is distinguishable on its face from the facts in your OP. You're not going to find the answer for your situation in Gates, just the overarching remedial intent of Sec 121.