Leaderboard

I just noticed that the titles are actually rendered in drawGuiContainerForegroundLayer() not render(). Also I believe you only need to set the value once in the constructor, or in init().

Then I don't know.

First, do not use IInventory. Use capabilities via IItemHandler. Second, look into the methods you are using. The maximum index those methods can read is 255. So, whenever index 256 rolls around, the lowest eight bits are kept making index 256 act as index 0 when reloaded and repeats for every 256 values (this is also known as a narrowing primitive conversion as referenced by $5.1.3 of the JLS). This is not to mention the load method bitwise ANDs the resulting byte received making the limit even more guaranteed.

Where will you be storing things at? Is it going to be saved per player, or itemstacks? Both of them can use capabilities. https://mcforge.readthedocs.io/en/latest/datastorage/capabilities/

What is an empty gui? If it does not store any information (e.g. inventories, sharing data) a simple screen will work using Minecraft#displayGuiScreen (this is client only). Otherwise you will need a registered container and screen, and open it with NetworkHooks#openGui (this should be done on server, in this case you send a packet when the key is pressed). There's a key input event that you can use, or use client tick event to proceed with keybindings (check Minecraft.java).