[LEAKYSEALS951]

Here's another 0.000002 cent idea:

It appears that the redwood is 5 strips glued together to make up a single plank, which were sanded flush, then the planks were then nailed to the wall side by side. They appear to be nailed to the wall with finishing nails which have been puttied over with filler. Looking at your original pics, I can swear I can almost see the original drum sander marks where these planks were originally finished. maybe, maybe not.

If the redwood is indeed pieces glued to make large planks, then it "might" make sense to see if you could remove the planks as the original large pieces, and take them to have them run through an industrial sized belt sander, which could effectively solve your problem right there. That would give you control of uniform reduction and no sandscratch. You get to re-stain them yourself out in the garage or something, and put them back into place.

The key is if you are able to get them off the wall in single pieces without damaging them. If you can get them off, this might go much quicker than trying to sand in place. I think if you were able to get one off, you might be able to coax the adjoining ones of by accessing from behind/side

There's my second wacky idea for the evening. If they could be coaxed easily, it might be easier and better than spending your summer sanding. I guaranteeif you do this, you will be amazed at how long even an industrial sander takes while all the while you think "thank #$% I'm not doing this by hand"