Chatham 4 ft. H x 8 ft. W White Vinyl Scalloped Top Spaced Picket Fence Panel - Unassembled

No, you cannot add another slat unless you also add longer posts at the ends of the sections. An additional slat will make the end posts too short to be able to be buried deeply enough to give the fence good support. You could purchase longer longer end posts and use them instead of the posts that come with the sections.

Yes but you don’t need an 8 ft tall post. The fence panel itself is a little less than 4 feet tall. A 6 foot tall post would be plenty. I’ve installed several of these fences using a 4“ x 6‘ Post

center pickets are 34 1/2"

The pickets have a 2-3/8 inch spacing

Ok so it matters a lot how steep the slope is and how long your fence runs. A gradual consistent slope run the fence at an slight angle to match the grade. The fence brackets allow for a smooth transition from section to section. Remember to run your posts at the same slope. We had a large drop off at the end on one side a sloped approach just made it look odd so it was run level.

There is one fence panel per box

You can cut a panel to the correct size you need and attach it to a post to finish the fence.

Yes the pickets are 1.5" x 1.5" x 46" (at the tallest point). The pickets snap into the horizontal rails, for more information on this process look at instructions / assembly guide under the "information & guide" section on the product overview tab.

t says the panel height is 46", but I assume that's the at the tall end. How tall is the shortest picket?

No. I didn’t. I just filled the bottom of the post hole with quikcrete cement.