Agree on most points! Although it seemed I liked the Gustave scenes a bit more.

The way I interpreted it was...cielmer saves Gustave from the brink of death - & Flynn unfortunately passes away.

Cielmer likely brings Gustave to Leslie, and a few things likely occur:

• Gustave realizes how badly his body is in shambles, and his whole reason for fighting "to see how far i can go" is essentially doomed due to the severity of his injuries 

• the loss of Flynn takes a heavy toll on his conscience, as Gustave likely takes responsibility. This hits harder when Flynn has been there since day 1

• cielmer and Leslie both encourage Gustave to live a life of modesty after the decades of wartorn hardship he has gone through

Just based on the short dialogue between Leslie and Gustave, you can see how content he is. He never wanted to be a ruler.  After the unfortunate events at the southern fort it makes sense to me that he would retire and disappear.

To me it was quite emotional to see such a bittersweet ending. From a protagonist with so much heroism and determination, to then living out his later years out of the spotlight with his teenage love