This should be feasible although not that easy (for a beginner)
First, I have indeed reserved some models and units (the so-called ExtraBrigadiers) for such a purpose, so your point 5) is right
Now, regarding the other points:
1/ Take the sample in the Graphics/Units and Graphics/Armies: it will give you the exact size / shape in pixels. PANGI or Nikel here are experts too
2/ To make a new leader, you need a model file (GameData/Models), a unit file (GameData/Units), and to reference both files aliases in the relevant alias files (Aliases).
3/ This is the most complex, as you first need to create your new leader's model and units files, then you need to insert it in a scenario. We mostly do so with the use of Excel files which are used to create setups (through a specific process that requires you to read in detail this forum sections on 'How to Mod Ageod Games'
). It's not so difficult, but may look akward at first glance for a beginner. You'll need help for your first try
4/ The file(s) numbers MUST be in sequence of existing ones, not random or 'what you wish'. Best now is either to use one of the Prussian ExtraBrigadier existing files, or add a file # which is the last of existing lists +1
May be start by finding the pic, making the graphics, and also writing here all the values / abilities you want for him