1) scripting.
www.nwnlexicon.com Put your script that checks if the item is in the inventory on the node. You can do this in on one of the tabs down to the bottom (it's the one left to 'actions taken' i forgot the name)
You need a script with a while loop which checks for all items with the tag you set.
2) scripting. Make a check script and 2 nodes. The one with the check should be the highest node in the tree, cause it needs to get checked. The other goes under this one without a check so it's shown if the check on the first one fails
3) yes, with scripting. Use the 'actions taken' tab in the dialogue window now and put a script which changes the XP in the slot there
BTW you can check with integers put on NPCs or the PCs or if you really need them to be undestroyable in a database.
I recommend you learn to script as you won't get much further easily and most often people won't write complete scripts but will help you if something you wrote doesn't work. The nwnlexicon has great tutorials.