In order to install mobile home skirting, you need to use the top moldings and the bottom track, which needs a base on which to be installed. Without those the skirting will look unfinished and the bottom will flap and move in wind. Something like the following railings adapted to below the porch would look much better. railings

Same way as the sample that houssaon provided. You don't want to mix wood with composite deck structure it will look out of place. You can also use lattice with 1.5" reveal Example Here, you can use Trex Decking material Example Here. You will also have to recut new stair stringers, and most likely add additional 2 stringers (depending on the width of the stair) to break up the span, composite should have stringers spaced 12" on center.Example Here

I would not do black. It will look like a dark hole. Here are some examples that would look good on your deck. I would use cedar and finish the same as the deck. As long as the wood doesn't touch the ground, it should last as long as the deck. I have cedar trellis on either end of my portico for over 20 years and they are just fine.

Perennials will dress it up and you won’t go whacking at grass which ruins the deck siding. I love Daylilies and a huge border of them would give you color, green and enhance your lovely new deck. Stella D’Oros are yellow all season, and come up early. Mine are three years old in this photo.