1. If a person missed a few fasts in Ramadan and haven’t finished making up all of their fardh fasts (but they intend to make them up soon), is it prohibited/permissible for them to take part in a sunnah fast (eg. the day of Arafaah or `Ashurah, 6 days of Shawwal, etc.) before completing the fardh qadha fasts? What is the fiqh ruling on this?

2. Is it permissible to combine two intentions for a single day of fasting, for example, if a person is fasting the day of Arafaah, combining the intention for a qadha fast and a sunnah fast or combining the ‘Arafah fast with the intention of Monday’s fast?

1. It is better to do fardh before doing Sunnah. That is priority. Fardh is obligatory, Sunnah recommended. However, if one reverses it putting the cart before the horse, it isn’t a sin, just missing the greater opportunity for the lesser.
2. Separate intentions and separate acts are better, but multiple intentions are rewarded where separate acts cannot be done, and even where they can be done.