Sunnah & Nafl Fasts

Fasting on Mondays

Q: An Aalim says: Traditional Eid-e-Milad-Un-nabi is Bid'ah. It is not Islamically right. But, it is true that Rasulullah (Sallallahu Alayhi Wasallam) himself celebrates the days Modays keeping Fast since it was his birth day, day of Hijrat, day of getting Nabuwat etc. I would like to know, if it is correct and if so, can we keep fast on Mondays like our prophet (Sallallahu Alayhi Wasallam)?

Receiving the rewards for fasting on virtuous days

Q: I made a vow that if I commit a certain sin I will fast for 3 consecutive days. Unfortunately I fell in to this sin many times after so I have 27 fasts to keep. If I fast with the intention of these fasts on days that are virtuous to fast like the first 9 days of Zul HIjjah, 13 14 15 of each month will I receive the thawaab that comes with normally fasting on these days or will only my vow be carried out?

Niyah for Nafl Fast along with Qadha

Q: I have learnt that one cannot make the niyah of a nafl along with a qaza sawm but for the sawm whose days are specified (e.g. Monday, Thursday, Yawm-e-arafah, Muharram, 15 Shabaan but not that of shawwal any that's any 6 days in the month of Shawwal), one can do qaza and one would automatically get the reward of that specified day if one has intended it as the aim is to spend that day in sawm, so just like how if one spend the nights of Qadr in doing Qaza salaat, one would get the reward of spending the night in ibaadah as the aim is to fill these auspicious nights with ibaadat. Please explain with reference.