1. You can't disable it globally, but you could place the <undroppable> note tag on all items in the database, but I realize that's not really the answer you're looking for. You probably want to disable the whole drop command window all together, right?
2. You can't make it 0 when equipped, but you could potentially grant bonus weight with the <equipWeight:+value> note tag. I could look into possibly adding this feature for you, though. How about an actor note tag like <equipWeightReduction:value>? This would subtract the value from the equipment item's current weight only when equipped.
I try to always be available to help with people's projects, but I'll admit it. I do get quite busy developing more plugins for the community and working on my own game. Others have commissioned me directly to get new features like this done quicker. I could also do it relatively cheaply if it were non-exclusive rights because I'd be allowed to share it with the community too. It's just something to consider in case I take too long for you. I'll just leave my email down below in case you are interested.
support@dmplugins.com