Giving one's own interpretation of the Quraan Majeed

Q: I went to visit a family member. They had some notes lying around. They told me they were going to do a tafseer class. I read over the notes and found some very strange explanation of the Quraan. 

When I looked in the tafeer, I noticed the notes were from Ma'ariful Quraan but they had added their own interpretation trying to say a Surah is promoting Dunya. 

Will a person who twists the tafseer to suit him be a kafir or what will his status be?

Questions related to a masbooq

Q: Kindly answer below questions related to salaah: 

1. If a muqtadi misses some rakaat, at what point should he/she leave the jamaat and stand to continue the missed rakaat? Should he/she do it after the imam completes the 1st salaam or after the second salaam? 

2. For a muqtadi who has to missed a rakaat in jamaat salaah, should he remain quiet after the last tashahud behind the imaam and wait silently for the imaam's salaam to continue the missed rakaat, or can he/she recite durood and prayers after the last tashahud? 

3. If a person misses the 4 sunnah prayer prior to Zuhr or Jumah Salaah and joins the farz prayer in jamaat, does he/she needs to offer the missed sunnah prayer afterwards or not? If yes, then what's the correct sequence of offering the missed sunnahs? Should those be offered immediately after the farz jamaat salaah or after the 2 or 4 sunnah prayers that we usually pray after Jumah or Zuhr farz salaah? 

4. If a person makes sajdah while a couple of hair strands cover half the width of his forehead, would the sajdah be valid?

Selling an item to someone not knowing whether they will use it for something impermissible

Q: 

1. Would it be halal for me to sell gold to a cash for gold place because there is a high chance they will sell this gold to a refiner, scrap gold buyer or so on and they might use this gold to make gold jewellery for men etc.? 

2. What about selling grapes to a business that sell grape related products so the grapes I sell to them can be used to make their grape juice or it can be used for their wine etc. Will the income be haram? Because I’m unsure what they will use it for.

Washing clothing that was soiled with urine

Q: My 3 month old son had a portion of his clothes wet with urine. I removed the onesie and placed it carefully on the bed with the urine part facing upwards so I could wash it. 

I fed my son and when I was done I picked up the onesie and went to wash it. Then I saw the onesie was completely dry all over and I couldn't find the spot where it was wet previously. 

Can I use this onesie now or must I wash the whole thing?