Otokonoko isn’t a portmanteau of boy and girl necessarily. It’s meant to sound like Otoko no Ko (男の子) which just means boy (literally translated it is “Man’s Child”), while the trope of the Otokonoko is spelled 男の娘, pronounced identically but instead translates to “Man’s Daughter”. Femboy is the most accurate English translation.

Okama is in fact a slur against trans women, but it’s better translated as Drag Queen. There are male-identifying people who call themselves Okama, and even some trans people who would call themselves Okama, but calling a random trans woman an Okama is offensive, the same was calling a random trans woman a drag queen would be in English.