It occurs to me there's a strong meta reason why Betty's dad isn't the murderer -- Betty is the archetypal girl-next-door, and to have her father premeditatedly murder her sister's boyfriend would do severe damage to the character and her archetype.
I think that ship sailed the moment Alice said she hoped Jason was rotting in hell. Betty's parents in this show are awful, twisted people, and Betty herself has kind of a borderline-deranged streak we've seen once or twice (like when she almost drowned a guy in a hot tub by stepping on his head and apparently thought she was Polly at the time). Which actually has some precedent in the comics, according to ComicsAlliance's running commentaries on the show. Apparently it's always been part of Betty's character that her crush on Archie can get kind of extreme and crazy at times, in a comedic sort of way.