Asalamu 3alaykum
Dear brother Student
I know these questions are directed to brother Joseph. I am only responding as he had mentioned his last post was to be taken as his last. However I do feel he does respond when he feels is right and when he can. So please allow me to make a little contribution to clear up some confusions you might have. However if I am in anyways incorrect I believe brother Joseph will disregard my response and answer your inquiries correctly.
The following Quran verses commands believers to spend and it is in imperative mood meaning it is a duty believers must oblige and take it as an obligatory act:
Yes obligatory as a voluntary act to believers. But not as a law or constitution. Allow me to explain
There is many men and women in the world who are blessed and trusted with huge amounts of money. No matter how that money was earned whether trade, inheritance, services etc. The money that is made at the end is what they own. Please remember this kind of possession as a different class of money than Zakat. I will come back to this and explain how this is to be used as Sadaqat and Anfiqu(spending).
In a country of an Islamic constitution, a law of an obligatory system of Zakat should be enforced and be part of the constitution. This system of Zakat is to be collected from the people irrespective of their faith or beliefs(41:6-7). The Quran also details on how to pay this due. It is to be collected from the people through their PROFITS and not their savings(6:141). The obligation here is that believers are expected to pay this with honesty from a religious perspective as well as an obligation to be setup part of an
Islamic constitution.
If you need more elaborations on this part please see the article called "WHAT IS THE CONCEPT OF ZAKAT FROM THE QURAN?"
Now once a system of Zakat is implemented the government will take their cut and the people will be left with theirs. The government will use that money for their own society and to strengthen it by whatever way they deem necessary. A duty upon the government to collect and use it correctly and a duty upon the people to pay it.
The above is all about zakat and not spending in the way of God or Sadaqa.
Now the verses you kindly cited are speaking to public in regards of their personal life however the believers who are entrusted with money of their own also have a duty which is to spend/anfiq. To spend/anfiqu in the cause of God. To spend/feed their parents, poor, needy, those who don't have work, etc. It can also be used to establish charities which from the money collected will be used to pay the people who work at the charities/people who collect(9:60).
How much should the people spend? Allah(swt) answers that in verse 2:219. The leftover/surplus/excess. This is all based on a persons ability. Everyone is different. Their is some people after paying their zakat/taxes who still acquire a whole lot of money and are wealthy and some who just make ends meet.
Those who don't have and cannot afford to give any money in way of charity or in the way of God are of course not required. However if they are making profits to make their living in their life they are expected to keep paying their dues/zakat to the government(58:13).
Also those who cannot afford to pay anything when asked, then depart with kind words.
Verse 2:219 suggest to spend as much as possible. It will not go to waste. This is in God's way. However their is also other verses which indicate how to spend(4:5, 17:29,etc).
Its arguably more of a blame on the wealthy who do not spend(9:92-93).
The verses you cited are for sure a caution to all those that have the means to spend. We can see that by what will be said on the Day of Judgement by the people who did not spend.
Those who spend is a great reward.
The Infaq/Spend in the above verses is referring to Zakat or something in addition to Zakat as you ruled Sadaqah out as an obligation?
They are referring to sadaqat/spending in God ways out of ones free will. Voluntarily.
I believe he had mentioned it is not an obligation like as a law from the government. It is not to be forced upon the people to pay Sadaqa.
Why a voluntary act is expected of disbelievers and presented as an argument and a case against them (36:47)
This is the response of the disbelievers. The greedy. The extravagant. Everyone is called on to spend from what God has provided for them. Those who respond like this of course are disbelievers.
Why would missing on a voluntary act becomes a source of eternal regret so much so a second life is pleaded (63:10)
Voluntary meaning given the will to act upon your own discernment on how much to give. Man will regret that he/she did not give as you see giving in the cause of God is great and will arguably remit ones sins.
Why would people missing on voluntary act aren't part of successful ones and presumably party of losers (64:16)
I think the above answers should suffice Insha'Allah
For the rest of your questions I think you should be able to make them out with the information provided Insha'Allah.
Also please brother Joseph if I intruded I apologize and omit/delete my comments if you feel they are not contributing or irrelevant to this post and start from scratch. I would not be saddened at all.
Salam