[] Plan: Advance along the Seas
-[] Decline
-[] Initiate an offensive toward Konigsberg (Gold).
-[] Initiate an offensive toward Zagreb (Purple).
-[] Deploy the Allied fleet to contest the eastern Baltic Sea.
-[] Ask the Allies to threaten the Dalmatian coast.
I don't think letting in Serbia and Greece is that helpful. Aside from the political considerations (which are significant), they're small countries, which do not have Alliance standard militaries, so they'll be much less useful than our forces. They'll also be taking 800,000 Italians from our main line to go fight in Macedonia, rough terrain that Greece will be limited in how well they can support them. And this mainly will distract the Bulgarian and maybe Romanian armies. Which I suspect won't be that useful to the League's main line due to coordination issues, especially the Romanians.
For our actions, I think now is the perfect time to take Konigsberg. I also think an offensive to Zagreb makes sense. The Austrians are desperate working on a defensive line to hold us, and Zagreb seems to be the weak link in it. If we let it go without taking it for longer, it will be further built up and possibly a harder target.
I'm also fine with this plan though, Russia is in a weak position and putting as much pressure on them as possible is good as well.