General Discussion

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It needs to be called global_loot_modifiers.json.   What you seem to want to achieve is to modify the loot in chests that have a specific loot table (e.g. simple_dungeon), right? Unfortunately this is not possible from a loot modifier, specifically because LockableLootTileEntity sets it's lootTable to null before generating, so if you try to grab it and check it it will just be null.

Thanks for the help, I deleted the KV mod and huzzah, minecraft finally started working again. Thanks again for your help!

General Issues 3. However, there are times this doesn't work as gradle is spaghetti at times.

I would say read the statements on the different kinds of sides prior to me explaining my reasoning. Even though all logic should be executed on the server side, sometimes it's less computationally heavy to execute the logic on both sides. For example, if your TE changes it's texture based on the amount of liquid it has, you might just want to send a boolean to client saying whether to increase the liquid or decrease it. It more or less depends on what information gets synchronized between the two sides. NBT data on an ItemStack is synchronized to the client when changed, so it should be logically sided within Item#onItemUse. Alternatively, Block#createTileEntity is called on both sides if we want to change how something like a TileEntityRenderer appears based on the stored data on the server. Same thing with entities except their spawning is sent through a packet to keep the same id. So, generally, all logic should be on the logical server unless you want to display something. Then, that information should be synced to the logical client. However, if it's sending a multitude of information every single tick, then the logic should be handled on both sides and synced via some value that switches both of their states. At least, this is my opinion.