The thing about the infinity stones is, at the moment, their overarching lore really doesn't matter, and that's exactly as it should be. Every stone that has appeared has been used as powerful mcguffins and little more. They operate within the confines of their own movies in order to make any kind of sense for casual viewers. The whole point of the infinity stone plot line in the MCU up to now has entirely been for the benefit of the hardcore fan. It's expanded slightly in every movie so hardcore fans can become more invested in each new film. This doesn't matter at all for the average viewer because it shouldn't. It's not for them in the first place. Much like the after credits scenes, they're bonus material for nerds to enjoy. It also has the dual benefit of enticing new fans to delve deeper into the MCU lore. The more invested they become, the more likely they are to see each new installment in order to keep up with the meta-narrative spanning the entire MCU. Criticizing the MCU for the infinity stone plot going over the heads of casual audiences misses the purpose of their inclusion entirely, regardless of whether or not the experiment works for any given individual.
You can bet your ass the stones will be summed up in such a way in Infinity War that seeing every single prior MCU stone appearance will not be a requisite to understanding what they are.