hi Lee,
well done!! congratz for ur gf having a visa..
about ur querries..
1. Yes Fiancee Visa is valid for six months..
2. Yes u should get married within six months or before her visa expires
3. why ur wife should return in the Philippines??for a vacation??
after u get married here in the UK, ur fiancee should apply for FLR (Further Leave to Remain) or 2 yrs visa extension, and its really a straight forward application..she dont need to come back to the Philippines to apply for another visa..hope it helps..