Deteriorating Deck in Mississippi

I'm so glad to have found this site. My situation is pretty confusing (to me) and this is all very new to me, but I'll try to be coherent.

1. State you live in - Mississippi
2. Full Shade, Partial Shade, or Full Sun - Full Sun
3. Type of Wood - New boards are pressure-treated pine. The original boards (about 6 years old) are a mystery - I'm guessing pine but not pressure-treated.
4. Mold or Mildew Issues - Several boards have rotted through and had to be replaced. Not sure if that's mold, mildew, something else, or if it matters.
5. Main Issues with Previous Stains - It has never been stained. We had someone spray something clear on it once or twice (I'm guessing Thompson's Water Seal or something similar) a few years ago, but boards have continued to rot.

Other considerations: The deck is attached to the upstairs level of a red barn, with stairs going up to it. The contractor who built this deck bragged about how he got the wood for practically nothing (or actually nothing, I forget). We didn't know we were supposed to do anything to it, and then boards starting rotting. Family were recently going to cover the whole mess with a resurfacer product (Olympic Rescue It Moderate) in a red color to "match" the barn. I was able to stop that from happening at the last minute, so now I get to do it myself. However, by this point the deck had been pressure washed and was ready for stain (or so we all thought, because I hadn't yet found this site). I went and got some Ready Seal in a light color, just to get something down and hopefully ease the family nerves a little bit. The newest boards were just installed a few days ago, and I know they're supposed to sit for at least a few months before staining, but I could tell that really wasn't going to fly even if it turns out to be a problem later.

The idea was that the resurfacer would completely envelope and seal the bad boards so that no more would ever need to be replaced and no other maintenance would ever be required. They were even going to paint the underside of the deck, which I've never heard of anyone doing. I feel like I dodged a bullet after reading about the problems with these products, especially in cold weather. We may be in the South but it does freeze sometimes. I decided after reading some reviews here that the Ready Seal was an acceptable (and immediately available) way to kick the can down the road while I research what should really be done.

I got one coat down yesterday on the deck floor. Was going to do the stairs today, but it is supposed to rain tonight and then freeze, so it will probably be Monday before I can do anything else. Now I'm reading that staining the new boards so soon could actually make them more vulnerable to rot too! They didn't seem to absorb much stain though, certainly not compared to the old boards which soaked it up like a sponge. I hadn't really noticed how rough they looked until I started applying the stain. I think the sponge applicator is actually catching little bits of wood that come off just from putting the stain on.

So, I'm not sure what comes next. Here are the choices I can see:
1. Add another coat (probably next week)
2. Add another coat, but only on the old boards
3. Remove everything with deck stripper and start all over (this will be the hardest sell on the home front)
4. Remove everything and start over, but doing something else to the old boards first (sanding? treating with something? no idea)
5. Do nothing and call it good until next year or so