I've got degrees in Latin and Greek and I am totally cool with the term polyamory. There are far more offensive misuses of language for me to get annoyed with. Polyamory sounds good in my ears.
Words that mix Latin and Greek are not uncommon. Examples: neonatal, sociology, television, homosexuality, heterosexuality, automobile, dysfunction, electrocution, etc. If these words don't bother you, then I wouldn't get annoyed with polyamory either!
The words "polyeros" and "multusamory," however, make me want to set fires and throw bricks, because they don't show an understanding of the original words and how to properly transition them into English words. Since Eros is a noun and the Greek god of "erotic" love, Polyeros sounds like he might be the Greek god of loving multiple people in a sexual way. Which is awesome, but not a good term for what we call "polyamory" in my opinion.
The problem with Greek words for love is that there are a number of them, each referring to a different kind of love. Using a word related to "eros" would imply that polyamory is "all about the sex," which the polyamorous among us would tell you is not true. The other Greek options, "agape" or "philia," are traditionally not very linked with sex, which is unfortunate because polyamory certainly does have a lot to do with sex. If I had to make the word "polyamory" entirely Greek, I'd go with "polyphilia" as the noun, and "polyphile" or "polyphiliac" as the words used to describe a polyamorous person. Although the original Greek word "philia" didn't have a ton to do with sex, it has been used in English for sexual terms (hemophilia, necrophilia, pyrophilia, etc.). Of course, polyphilia sounds like a weird fetish that might get you arrested.
"Multusamory" is bad word-making for reasons I won't go into in this forum (ask if you're interested). I'd go with multamory, multamorous, etc. or pluramory and pluramorous. I like pluramory better.
Obviously nobody is going to change the term polyamory, and I'm happy with that because the alternatives leave something to be desired. But, there's the Classical Language Geek take on it.