The vanilla shield actually has two model files that it uses. minecraft:item/shield.json and minecraft:item/shield_blocking.json. You need to use the ItemOverrider you have in the constructor to tell it to point to the second model.
Trying to start ANY pack through the twitch launcher, with or without any mods fails to start minecraft with no real error code. logs are provided, pack had no mods in it to test if it would launch