Well, I can help with at least a few of these. Mind you it gets pretty complicated. GE (Galaxy Editor) is very similar to WE (World Editor) but it's a little different and broken up into more pieces.
1.) This one, if I understand you correctly, is pretty simple. To remove an ability from a unit go to the data editor (F7), this should come up with another window with a listing of all the units. Browse to the unit you wish you remove an ability from, click on them, and on the right will be a whole lot of fields that control the stats of that unit. The 2 fields you want are "(None) - Abilities +" and "(None) - Command Card." Those are where you add/remove abilities.
2.) This is similar to what I said in #1 but it gets more complicated. There are some stats where I talked about in #1 but to get to the dmg/what it can hi/range etc you need to go somewhere else. at the top left of the data editor there should be a selection box called "Data Type." If you click on that and scroll down to select weapons a tab pops up where you can modify the various weapons used.
For example: in the weapons tab browse to Terran -> Marine - Gauss Rifle. If you select that the nyou get some more stats to play with, like the rang of the weapon.
If you want to change dmg then go back up to the selection box, scroll UP to effects and select that. In the effects tab browse to terran -> Effect Damage -> Marine - Gauss Rifle (Damage) and you can change the properties there.
3.) I haven't had much luck here yet though I have not looked much into it, sorry can't help you yet.
4.) I'm not exactly sure what you mean here so for now I'll skip this
5.) Creating an ability is fairly complex, much more so than in the WE. I could explain it but that would take a whole lot of time (it really is annoyingly complicated, unless I'm missing something crucial) which I don't want to spend quite yet, I'm currently trying to make a map of my own.
I hope this was helpful in some way and that I explained this in a at least semi-understandable way, I'm not a very good teacher.
Cheers and have fun playing with this amazing editor.