Everlys were an influence for sure.
But the Beatles rocked more, check out their Hamburg recordings.
John Lennon himself stated nothing affected him til Elvis.
Other obvious influences are Chuck Berry, Little Richard, Motown (McCartney said Motown was the biggest influence on his bass playing).
The were influenced by Buddy Holly and even were inspired to use Beatles as their name as a play on the Crickets.
They covered a Holly song, Chuck Berry, Berry Gordy, other Motown.
Not sure they ever covered an Everlys song.
I say they had so many influences none can really be said to be the biggest.
In the first place, you are recycling pieces of conventional wisdom. We’re talking about vocal harmonies. Not only did the EB influence the Beatles, but the Hollies as well (who even played on the EB albums recorded in the UK.) Both groups admitted it, and probably in order to avoid comparisons, refrained from recording Everly Brothers songs, which were more country oriented than either of these groups. There is nothing ‘debatable’ in this.