If Endymion had never met Serenity...

[SailorStar9]

Could he had fallen for Beryl?

[Crescent Pulsar S]

Who knows. We know next to nothing about Beryl prior to being under Metallia's control (her personality, social status, et cetera), and absolutely nothing about the circumstances in which Endymion and Serenity met. For all we know, all Endymion needed was to see a picture of the princess to catch his interest, rather than actually meet her first.

Even if Beryl were in a position to have a relationship with Endymion, seeing it realized might be dependent on when that happens, since Metallia's apocalyptic machinations (even if Beryl avoids being directly controlled by them) are bound to make the forming and/or keeping of a relationship impossible.