The visa expiration date will likely match the medical expiration date, and travel must commence before the visa expires. Guam is a POE, but once she goes through there, the K-1 would not be valid if she returned to the Philippines and attempted to exit a 2nd time using the K-1. It is single use.
Flying out by the Feb 7th with an interview on Jan 18th is possible but cutting it pretty close. She has to wait for visa delivery and then attend CFO. How long the visa delivery takes depends on where it will be delivered. Once those are clear she is free to travel.
You do not need to wait for the welcome letter for Manila. You are free to schedule your medical and interview. The welcome letter often doesn't even show up.