Why no Froggie Beastlords?

[Troodon]

IMO half elves are like mules. They are sterile (if they could be children of either they would be able to mate with either)

There are no mention of any 3/4 elves or 1/4 elves in the game prooving my hypothesis

That doesn't prove your hypotehesis. Absense of evidence is not evidence of absense. It could be they exist but are just not mentioned. It's also quite likely that any of partial elven heritage just call themselves "half-elves" regardless of the precise ratio.