What Barney said will probably work fine, since you seem to have a pretty solid plan.
I can give two answers to your questions though, but please keep in mind those things work for me, not necessarily for you too.
1. I never ignore the Siberians. I kill them, chase them down, or screen/defend against them, depending on what Russia did and what my own plans are. Since Russia moves before Japan, I adapt accordingly. Sometimes it’s even worth it to allow them 1 or 2 turns into Manchuria/Korea if you can kill them/force them to retreat later on anyway (no later than 3rd turn after), because this keeps them away from Moscow and ‘upsets’ Mongolia.
Good thing to know: Japan can take down China + India completely without the troops from Manchuria/Korea. IJAF has 21 airplanes and it’s OK to sacrifice ~4planes if this means killing the entire Chinese (or Indian) army.
2. Use them when you can ;-). Use them to deny USN shorebombardment or to strengthen the IJN if it comes under threat of an attack so stage it off Japan and/or Philippines.
I usually let russia go to korea, but make sure I can deadzone manchuria. Especially good idea since korea is easily convoy raided.