Royal favours in Caylus
Rules reference here

1)  No, you may not build another building.  As you are aware the rules say: -


  If a player gains several favors during the same phase (for
  instance, during a count or when the monument is built), the
  favors must be used on different lines. So, it is impossible
  to gain more than 4 favors in a phase.


You gained the favour allowing you to build in phase 6, building the castle.  If you build a building that give you more favours the phase has not changed.

2) Yes,  You advance your marker unless prevented by the stage of the game (ie building the keep, walls etc).


  Whenever a player earns a favor, they must choose a line at
  once, and, if it is possible, advance their marker to the right.
  Then, the player can use one of the effects provided on the
  chosen line, between level 1 and the level currently indicated
  by the marker. 


The restriction on moving the marker is the stage of the game, not if you are able to carry out the action.  Don't forget when moving on the favour track you don't have to carry out the action you move to, you can select any of the action you passed on that row.  Also if you are the end of the row you can still keep selected that row and carrying out any action.