Strip/Sand or Replace?

Hello, I have a ten year old pressure treated deck in Omaha.  It's about 14' x 14'.  When I purchased the home I used a solid stain, and a few years ago I sanded it down and applied Behr semi transparent, which began peeling within six months.

My home has a walkout basement so there is an outdoor dining table under the elevated deck, so you see the bottom.  The bottom of the deck and railings are a dark red/brown solid and the decking is a pale gray.  The decking was installed with nails and several pop up each year.   A few years ago I replaced the stair treads with one 12" board each (stringers were breaking off at some tips so two 6" boards wouldn't hold).  Otherwise there is only one rotten board and several warped spindles.

Anyway, I'm ready to tackle it again and want to go with TWP 100.  My questions:

• Should strip & floor sander or should I just pull all the decking boards and replace with new pressure treat?  The actual board cost would be about $300.  Maybe it would be faster and have a better end result?
• If I sand I should probably pull all the nails and replace with screws or I will tear up all the paper, correct?
• If I strip/sand, is there a good way to remove or cover up the pale gray Behr semi from in between the boards?  It won't match the red/brown underneath or TWP 100 on top.
• If I strip/sand I'm assuming just re-coat the railings with brown/red solid as they look decent (some black spots, mold? and some where it has just worn off, no peeling that I see)?
• If I replace the decking would be easiest to also replace the railing with an aluminum or PVC?   My railing posts are 6x6 currently, two of which go all the way up from the ground through the railing, I would want to make sure I can put something sturdy up that won't wobble/wiggle.

Thank you for your help, I'm wanting to do the best job for the long haul, if it is just a couple hundred $ more but saves me a lot of time I'd prefer to go that route, but most of all want the way that will last the longest and be easiest to maintain going forward.