Why not just use a second account?
Would THIS be what you are looking for?
If you go to 4 minutes and 6 seconds into that video, you can see that he uses forge with 8 mods loaded. Also, Skeppy has made video's before...
I am going to say that this is not possible without a mod (or at the very least maybe a resource pack, but I doubt that) .
Maybe my plugin PermissionShopZ will suit your needs.
This plugin claims to work with Crackshot, maybe you can give it a try.
Awesome that you learn kids how to program trough Minecraft! I'll create this plugin for you!
EDIT #1: I assume that @Jamiezi did not continue...
Where would the plugin take the diamonds from? Would they already be in that players inventory?
Pasting it into the sign editing GUI is client sided. It would only be possible with a command or anvil GUI.
To answer the question:
Someone would have to make a plugin that uses the api of the register plugin to know when to save/restore the items...
You can force it, but you can stack it so that the server crashes.
You mean like this: https://www.spigotmc.org/resources/13571/ ?
Sure, there are ways to make it work with a custom server and maybe a custom client.
That depends on what you mean by "your inventory is updated". Do you mean when a player opens their inventory? Or maybe when they receive an item?
The reason I did not wanted to do that is because there are several reason a player could disconnect before re-spawning and receiving their items,...
Separate names with a comma.