Newbie question related deck staining

We bought a new property and it has two decks. One is close to 15 years old (the original one) and there was an extension made on a later date. No idea when the new one was done and what stain was applied to it or none at all. The old one was painted in grey and I believe it was Behr deckover (as it is a royal pain in the backside to remove it). The new one was stained and the stain has lost the effectiveness! Both the decks needed attention. So, I decided to remove the deckover to stain both the decks. Luckily all the railings are fiber and white. So, no issues there. The first picture is of both the decks in the original condition.

For the older deck with Deckover it took two application of stain remover and I really brushed it very well each time with a hard brush and followed all the directions and used a pressure washer to remove the paint. After the whole exercise, it still has a lot of old paint remaining on the deck. See picture 2 & 3. I am now sanding the deck and plan to do a beck brite before I put the stain.

I have a few questions: 

1. To what extent do I need to sand? There is deckover in the crevices, between the slats and also embedded in the wood fiber. Removing all these by sanding will be a tough job. Do I do another round of stain remover or the deck bright will take care of it?

2. What to do within between the slats? The previous owner really put the deckover with great detail and it is there in between every slat. Not sure how to remove those as these are hard to reach.

3. If I put a darker (coffee or similar) stain, is that going to make the decks sort of similar looking? I know it will never be similar.

4. does anyone recognize what kind of wood are these?

Pic1: Both the decks together

Pic2: After 2 rounds of stain remover and pressure washing (deckover)

Pic3: Newer deck after pressure washing

Pic 4&5: The old deck in process of sanding