Hi!
Thank you very much for the details, it helps me a lot.
Basically, what you need is to change the keyword logic to ‘AND with exact keyword matches’, and I am pretty sure it will do the trick.
This logic will only match indexed keywords, when the whole word entered is strictly matching the indexed keyword.
Also, both methods you used for storing the additional keywords (search meta box and the product attributes) are perfectly fine. I recommend using only one of them, as they are redundant. Maybe using the product attributes field, if you have any other plans with them, otherwise the meta box is fine.