I'm not much help with most of your post, but I can tell you how to breed Synthesis onto Oddish (although Moonlight, which Oddish learns naturally, does pretty much the same thing anyway.) In Silver Version, there are three pokémon that learn Synthesis naturally and can breed with Oddish: Bulbasaur, Chikorita, and Sunkern. Sunkern is probably the easiest choice simply because you can catch them in the wild. However, if you have Crystal Version, forget Sunkern. Hoppip, which can also breed with Oddish, learns Synthesis naturally in Crystal, and it learns the move extremely early, so you don't have to raise it first.
Oh, by the way, if you're stuck using Sunkern: don't evolve it. Sunkern and Sunflora have very different move lists, and Sunflora doesn't get Synthesis.