I'm not the best modder, but I've done it before so I can probably help you out.
raw -> objects -> plant_new_trees
1. You can't gather leaves of trees in fort mode yet. Sorry
2. [GROWTH:FLOWERS]
[GROWTH_NAME:immature octagon cluster:STP]
[GROWTH_ITEM:PLANT_GROWTH:NONE:LOCAL_PLANT_MAT:FLOWER]
[GROWTH_DENSITY:1000]
[GROWTH_HOST_TILE:BRANCHES_AND_TWIGS]
[GROWTH_TIMING:60000:119999]
[GROWTH_PRINT:5:5:7:0:1:60000:119999:2]
[GROWTH:FRUIT]
[GROWTH_NAME:octagon:octagons]
[GROWTH_ITEM:PLANT_GROWTH:NONE:LOCAL_PLANT_MAT:FRUIT]
[GROWTH_DENSITY:1000]
[GROWTH_HOST_TILE:BRANCHES_AND_TWIGS]
[GROWTH_TIMING:120000:200000]
[GROWTH_DROPS_OFF_NO_CLOUD]
[GROWTH_PRINT:'%':'%':4:0:1:120000:200000:3]
[GROWTH_HAS_SEED]
that's from coffee, it should help.
3.
frequency?
4. reaction? yes. Bark? No.
5. items? you mean aforementioned bark? But the fruit/leaves absolutely! You just need to know how to color it in GROWTH_PRINT
6. Material templates. man. Local_plant_mat:Structural. I think you'd need to assign it a new template and give it new submaterial templates. if that makes any sense... (I'm not very good at this myself.)
7. Look at highwood. [DRY][SAVAGE] and [BIOME:NOT_FREEZING] tags on it.
Hope I was some help (I wasn't.)