I don't think you're limited by the spells you can learn as long as you take the time to comprehend them. Like Tang Wei had his own Dao, but it's very likely that he had knowledge of spells from other Dao specifically. He hated imperfection and Arts but I'm pretty sure he still knew a lot of them, being him.
If you just start out at the beginning of the game, you can theoretically learn any Art right? It doesn't have to be Void, Elemental or Body, which are all Tang Wei's personal versions of the Dao these concepts represent. LB even says at one point that no two Dao are the same, so your personal Dao is your understanding of the "truth" behind that concept, and two Fire Dao practitioners that live the exact same life would still have different versions of the Fire Dao.
She also says that Arts or spells are just systematized and simplified formulas that allow you to use the Dao used for their creation in a limited controlled way that makes it easier on the mind and body. So I'm pretty sure that not all spells present in PoMA have to be derived from Tang Wei's Dao. For example all the mind control effects, dice rolling, the beast summoning guy from the arena, or all the poison techniques (although it could be argued that it's a very complicated way of using elements but whatever). However that they come from Tang Wei is still very logical, since they didn't have to be his for him to create manuals for them or teach them to people, he just had to have comprehended them.
And yeah, it's stated multiple times that he abhors all inferior techniques, but since he was drained, on the verge of death and Qi starved, he probably resorted to some old knowledge he had on "basic" and "simple" Arts he knew for whatever reason.
Or maybe you are right and Niki has once again decided to just retcon everything - both are equally as likely lol.