1) I'd slope from front to back, but that's just me. The substrate flattens out over time so you might want to slope greatly. Looking at pics of Amano's tanks of the sides, his substrates are absurdly sloped.
3) I'd say Riccia grows faster, but glosso is pretty near behind. You can't get less maintainence with either of the two plants
4) Yes, Amano focuses on the aquascape most of the time. He has mentioned that fish are usually chosen for complementing the scape, not the other way round. And imho, yes, discus will ruin the layout.
5) My take is, go without a background if your water is really clear. Otherwise, black usually works.
6) I agree that it'll look better.