1. yes it will be stronf enough
2. yes i think so- rather than persplex you could use rocks to make it more natural, if you siliconed lots of slate rocks togeter to make lotsa small waterfalls that would look pretty cool imo.
3. i like the l shaped, if you are putting it in the corner of a room. if I was having it against a wall i would personally have 3 sides planted. (a c shape)
4. for growing mangroves i would reccomend growing them in plant pots with lots of holes in (those plastic ones are perfect.) you would probably be best running a 150watt metal halide with a 6500k bulb in for mangrove growth and of course you would need to wait until you have converted the tank to brakish.