3 - Thats not what developer is for. Surprisingly its for the "developers" to use not the players
4 - "developer_script" definitely wont benefit him at all, as all it does is enable the developer code blocks which is to assist with developing "scripts" not fixing errors
just /map thename isnt gonna work unless you load the mod - and from your description it sounds like it crashes whenever you actually try to load the mod
the problem you are having isnt the model, its the way you are overriding assetsbecause this is a duplicate of a stock gun, its loading the "assets" from the stock file, then in game - reading your file - thus trying to use the new model - but the "old one" is all thats loaded
Your getting these errors because you did not install the patches and updates for mod tools correctly. Fixing the error, just means you will get "another error" from the "next" set of files that are missing
Reinstall the mod tools - and this time do it "correctly"