Is deckover or a simple stain more appropriate for this deck

I'd start by LIGHTLY power washing with a detergent, or if you don't want to rent a power washer and need a workout, use a stiff bristled scrub brush to scrub down the deck with detergent. That'll remove the black/slimy stuff, and will probably resolve most of the problems with the deck itself. Don't really grind into it with the power washer.

The main thing you're going to need to figure out before you can replace portions of it is precisely what the material is. Did your parents have the deck built, or was it there? If you can find old receipts or quotes, that might tell you what the material is. It also might be under warranty -- most of the early composites had a warranty for 15 or 30 years, and if it's only ten years old, then the spongy boards might be replaced by the manufacturer if they're still around. Hard to tell without actually having a product and manufacturer name.

The difficulty with replacing boards in a deck like this without exposed nails is that you have to take off most of the boards to get to the one you need to replace. You start at the edge and remove the facing board, and then you have to remove the hardware holding each board down in turn to be able to safely remove and slot in a replacement. The replacement also won't match, and unlike wood, the composites don't weather to the same color quickly. If the money's there, you might just consider having the entire deck resurfaced, especially if your parents are elderly and the house is going to be sold as they move on in life.

You can likely answer your own question with a little experiment. Before committing the whole deck to large amounts of stain application try a small area on 1 or two boards where you've sanded on one side and the weathered grey is still on the other side. If you like the result you will know what to do. If you dislike the look or match then you can proceed to close up the stain can and go back to sanding and scrapping the rest of the deck surfaces. The place where you put the stain sample over the weathered grey can be sanded down to fresh wood right along with the rest of it.