A- I think the placement of the fissidens on the root relative to the other moss is fine
B- I would do blyxa japonica, because it looks like a shrub, if you are doing a prairie field with a tree there should be some bushes.
C-I would get the Finnex Ray 2, best bang for you buck and 8 hours a day is a good amount!
D-Keeping it on can create a better balance, just make sure to monitor your KH levels as long as the saturation of Co2 in the water stays lower than the KH levels you are fine. Get a drop checker if you can. Green drop checker= 30ppm C02. 2.8 DKH = 50ppm. You don't want KH lower than 30ppm, because you will run the risk of a horrible outcome.
E-I think it is a cool idea, I would add another smaller tree or bush, doing the same method under the bigger tree, just to the left of it. Or a smaller tree over in the back right corner.