I also have a question: How does the alchemist ambassador works in the solo game?? Just move the ambassador onto a place which is occupied by your own seward?! If that's not allowed, the alchemist ambassador wouldn't work in solo mode...
The rules do say "at least one other player's Steward", so it looks like you are right about the alchemist ambassador not working in a solo game - I can't find any rules exceptions to cover it. I guess the same is true of the labourer ambassador?