You are entirely correct, and it has since been clarified what happened by 7.31b: Zombies used to do the basic damage type but didn't have their damage adjusted when it was removed and all damage was made the 100%/100%/100% hero type, which was 33% buff in damage for them.
The first game (Returns) is good but rough, with nice story and atmosphere, but the combat is clunky and stuff like hacking is downright maddening. The second (Dragonfall) polishes stuff up and is real good. The third (Hong Kong) is simply all around great.
If you're curious but not that interested just jump into Hong Kong. In principle the games don't story-wise spoil each other, but I honestly think it'll all feel too clunky going backwards, but rather that people only play Hong Kong than miss out on that too bouncing off Returns.