Yes you can go to Bulgaria with romanian visa only for a transit and for a maximum stay each entry 5 days on your way to Romania.
To enter in Romania with a passport from Philippines you need the Romanian visa. See the links below for details.
Yes, U.S. citizens need a visa to enter Bulgaria for stays longer than 90 days. For short visits, such as tourism or business for up to 90 days within a 180-day period, Americans can enter Bulgaria without a visa. However, they must have a valid passport and may need to meet additional entry requirements. It's recommended to check the latest travel advisories and entry requirements before planning a trip.
NO.
no
no
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No you cannot enter without a valid Romanian Passport
A Romanian citizen can travel in European Union countries only with the ID.
No
list of visa free countries for bulgarians
It is highly doubtful.