Qadha Salaah

Q: Can one read voluntary prayer if one has missed prayers to make up?

Q: If my deceased parents have not been steadfast with zakaat, salaah and fasting due to reasons that I am not aware of, am I able to compensate for this, even if they have not mentioned it in their will? I would love my parents to be in Jannah, so how can I help them achieve this now that they have passed away?

Q: If a person is offering qadha of Fajr of the same day, then should he offer the sunnat first and then the fardh, or vice versa?

Q: If I have qadha salaah to perform, can I perform it after Asr salaah?

Q: I have qadha salaah from a certain year. I have read salaah in between but I'm not sure when my haiz fell during the year as well as which salaahs I read.

My question is, my intention for the qadha should be made from a specific date that my salaah became obligatory and each date so on till the current date, or should my intention be that, from the first of all the qadha I have right up until a number I assume I missed. 

Q: Regarding qadha salaah (missed prayers):

What is the full intention to make before one performs a qadha salaah (Note: the intention for ample salaah missed not just a single salaah). I am fearful that if my intention is incorrect my qadha salaah wont be accepted. Many books have the intention for only missing one prayer. 

To my knowledge the intention is "O Allah, I am reading the 1st fajr of those that I have missed" but I am unsure if it is correct.

Q: What is the method for qadhaa namaaz and fasting?

Q:

1. I have a lot of qadha salaah and I don't know exactly how much, how should I calculate all my salaah?

2. While performing my qadha salaah, can I perform all the Fajr, then all the Zuhr, then all the Asr and so on?

Q:

1. If I am offering my qadha namaz and iqamah is called for the next prayer, do I have to stop praying qadha?

2. If I prayed full qadha namaz in this situation, is my qaza prayer valid?

3. What if I am just about to complete my qadha prayer and iqamah is called for the next prayer?

Q: I have nafil and sunnat prayers to make up that I used to break in the past and are now wajib. Is it valid if I make the niyyah of "2 rakat wajib"? Or do I have to specifically make niyyah of sunnat/nafil and which daily prayer they belonged to?