So what is the proper plural form? Felies? Felixeses? Feli?

I believe it's "Felixies". Just like the plural form of "fox". "Foxies"

Felix Felicis

And this is how the English language is borked.

I would go with Felii... to quote Jim Butcher in Harry Dresden:

“I am the foremost collector of velvet Elvii in the city of Chicago," I said at once.

"Elvii?" Marcone inquired.

"The plural would be Elvises, I guess," I said. "But if I say that too often, I start muttering to myself and calling things 'my precious,' so I usually go with the Latin plural.”