If you want to use a command and the user isn't in your server, you can enter their user ID as parameter instead of their username.

To find a user's ID, you need to enable Discord's developer mode by clicking "User Settings" > "Advanced" > "Developer Mode" slider.

After enabling developer mode you can simply right click their username and click "Copy ID" button.

If you invited Jim after 24th of March, 2021 you need to kick him and invite back for slash commands to appear in your server.

This is due to Discord grandfathering servers that already invited at that point into having slash commands permissions for Jim, any server that invited Jim afterwards lacks the permissions Jim needs to display slash commands.

No, for some commands you need to have a specific permission.

• Ban command requires Ban Members permission
• Softban command requires Ban Members permission
• Hardban command requires Ban Members permission
• Kick command requires Kick Members permission
• Mute command requires Manage Roles permission
• Warn command requires Kick Members permission
• Clean command requires Manage Messages permission
• Modifying guild settings and tags requires Administrator permission

Jim uses pure discord permissions to handle muting users, it creates a Muted role and creates permissions in every channel so that users with Muted role can't send messages. To make sure this works properly

• Make sure Jim has the permissions it needs
• Make sure Muted role is above the highest role of the user you are trying to mute
• Make sure your channels have proper permission overrides for the role Muted