That's a bunch of questions i thought were explained by the initial post, but i can understand that it's hard to fully grasp, lemme try to break it down some more.
1- The stats on the table are the final stats you get by equipping those unit sets, before any affix. That means combined total defense value + each invidiual units hidden stats + the set bonus if there is one.
The main thing to consider is here that 11* and below mostly gain their stats through set bonuses (having to equip multiple parts to get a bigger chunk of stats) vs 12* that mostly rely on individual hidden bonuses (like Ray for example). There are some units that fall out of that system like P-Drive for example which is a singular 11* Back units with strong individual stats, but it lacks a corresponding set.
2 - The idea behind P-Drive is simply that it is a very strong unit on its own and combined with a 2 Set bonus or other individual strong units (like Ray) they reach a decent net total in stats.
3 - That is exactly what i tried to bring across with the unit list. Gemini Set (2 parts) + P-Drive (1 part) or Ray Units (2 pieces no set bonus) + P-Drive. These are mostly mix and match options, and looking at the final stats, they don't differ too much.
4 - All the unit combinations marked in red are my "recommendations", which mean i consider them close enough to each other in terms of power level that i consider these good enough. The Gemini + P-Drive would also be red, but i forgot that and will probably change that once i add the invade set to the list.