70% done sanding deck, please see pics and questions...

I initially tried to strip but was unsuccessful as the Behr semi-trans stain was like paint - just didn't want to come off.

I am now in the process of sanding the entire deck with a 5" ROS and a palm sander using 60 grit paper and I think its going well, but I have a few questions on the current state of the boards:

1. There are some boards that look brand new (first pic), but there are a lot that have grooves in the wood where the old stain is sitting in and I cannot sand it off...
a. should I keep sanding these areas ​and (possibly go down to 40 grit) to try and smooth out the grooves and get the stain? Or will I be OK leaving these boards as is and will the new stain look good? See 2nd pic showing boards with minimal stain remaining in grooves. See 3rd pic of a closeup of the grooves in a lot of boards.
b. should I try to now strip again and really soak the stripper into these grooves to try and get rid of the difficult to reach stain? Damn Behr stain! I am using Defy, would a Behr stripper work better on Behr stain?

2. I am using a solid white stain on the verticals, so instead of sanding these 100%, will the current state be perfectly OK for solid stain application? I gave it an easy sand but don't want to spend 10 more hours sanding the spindles and horizontal boards across the spindles if I don't have to. Repeat question but showing a pic of current state just to confirm. (4th pic in the next reply).