Is the fast valid if it becomes apparent that Fajr dawned before finishing suhoor, given that the fasting person relied on a calendar that was later found to be incorrect? Is it permissible to combine the intention of making up a day from Ramadan with the intention of fasting the White Days in Rajab?

Since you had your suhoor (definition) at 4:30 and it became clear that Fajr begins at 3:41, your fast is invalid. As for the validity of combining the intention of making up missed Ramadan (definition) fasts with a supererogatory fast, scholars have differed on this matter. The more prudent approach for someone who has missed fasts to make up is not to combine any other intention with it.