With regards to Vesela and Mathys, though they might appear in routes where you are already romancing one of the other characters, they are only available to romance in the 'single' route.
Currently, MC that are married to Guillaume, but not in a romance with him will only be able to romance Guillaume. This may change as I write, but romances will still be limited - not many would tempt the royal family's wrath by having an affair with the crown princess. ^_^