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Fluid models rendering with gaps was a known issue that was fixed in Forge 1.11.2-13.20.0.2279. It's very unlikely that this will be backported to 1.10.2. The model loading for the inventory variant not being loaded is probably due to you adding a blocks/ prefix to the model path. I'm guessing your blockstates file is assets/sw/blockstates/fluid/plutonium.json, not assets/sw/blockstates/blocks/fluid/plutonium.json. You can use forge:fluid as the item model if you want the item form to render as a 2D-style square with the fluid's texture. Having the item form look like a bucket may be confusing, since it wouldn't actually be a bucket.

This is basic Java knowledge that you should already have before making a mod. Store the EntityCow instance in a local variable, set the fields and then pass it to World#spawnEntity.

The GuiContainer#inventorySlots field stores the Container that you pass to the GuiContainer constructor. You could have found this out for yourself by looking at the GuiContainer class in your IDE. Slot IDs only have to be unique within the Container instance. Two different Container instances can each have their own Slot instance with ID 1.