I will create spigot minecraft plugin for you

Unfortunately no

As stated on the packaged, though there will be times that I won't be able to meet those deadlines depending on the complexity of the plugin.

No, I will create this plugin just for you and you only. Since it won't be fare for you if you pay for this and other people can just download it free if I posted it.

if the bug came from the plugin I created then Yes, I will fix this, but if other plugin is causing the bug, then I won't be able to fix it.

This will still fall on the Advance Plugin, I can still do this but this will still depend on the Idea, as much as possible I want to offer a lightweight plugin that won't depend on other plugin. If the idea is very complicated and needs to use NMS then unfortunately I won't be able to do it.

Yes I already created an RPG plugin some of the features are.. A leveled mob system, the more you go away from spawn they will increase their level, health and damage then some custom enchants and new skills.