Yeah, that’s what I was going to say, too. All unstressed vowels tend to merge into schwas, and can be interchanged without sounding more than a little wrong.
And just as a reminder, the plural of “penis” is either “penises” (according to English rules), or “penes” (according to Latin rules). Like most Latin words ending in -is, the plural is -es (or at least, the plural nominative, but English doesn’t use different forms for different cases of amateurnouns). Using something like “penii” (which should actually be the plural of “penius”, if there were such a word) is both pretentious and wrong, the worst of both worlds.
I’ve generally thought that the word “penis” absolutely must come from the same place that “peninsula” does given the fact that the penis is like a peninsula of the body. The latter comes from Latin “paene insula”->“paeninsula”, meaning “almost island”. But the similarities are likely just a coincidence as Wikitionary lists different PIE roots for “penis” and “paene”.
In my dialect, the second syllable of penis is a schwa. In fact, the large majority of unstressed syllables are the same. We have no standard way of representing this. In addition, what is a schwa in my dialect might not be in yours and vice versa. With only 5 or six vowels in the alphabet and 22-24 distinct vowels in various dialects (a range I read somewhere; I never tried to count them) it is hopeless to represent all the sounds in spelling.
Not so much entered the language as frozen after Gutenberg. Before printed books, spelling danced all over the place. Even someone writing their own surname could spell it varying ways, sometimes in the same document.
We’d also lose the typo: the penis mightier than the sword.
