And trying to build a sort of raider.io mesh would be a nightmare. Last time we had such a mesh network of add-on communications was oQueue, and Blizzard admitted it was causing server side stress.
Offline download is the best. The client gets all the data in one easy to download file.
WoWUp and all other alternative clients are fully open source. You can read through WowUp’s source code yourself and see exactly what the program is doing. If you don’t trust the version they have up for download on their site, just build the program from code yourself.
Overwolf wouldn’t open source their client in a billion years. There’s no way to know all the crap it’s capable of doing.