[onewhippedpuppy]

That deck looks pretty good, if it were me I would replace any rotten boards, screw them all down, then pressure wash and re-stain.

I have composite decking, it's all 16" OC and isn't spongy or sagging. I would not do a scarf joint where the panels join, assuming this is the hollow decking you'll lose all of the vertical strength of the composite "board". When you cut the composite stuff at an angle the sharp edges really tend to chip and bend, I think over time the join wouldn't look very good. You'll also loose most of the material to screw through. I would butt the seams like a normal deck, worst case you might have to add another 2x nailer underneath.