Now...why The English language decided to go with octopus instead of octapus, i have no idea!!!
I detect a healthy use of Liddell & Scott with some Goodwin and Gulick thrown in as well. I believe in researching the word you will discover that, as you note, the original derivation of the term is clearly Greek, but the Romans later Latinized it to “octopus” which served as the basis for the English language term. Considering that to be the case, the plural form in western English would be derived from the Latin first person plural nominative case, “octopi”. It’s a Greek word which, when Anglized, gives the nod to the Latin speaking Romans and not the originating Greeks. Romans liked to appropriate stuff and claim it as their own without giving credit to its original source. They felt they had the bigger army.