I think they can leave, but they need a green card and a passport to return to this particular area. Its not an open two way street here. Its the best way to put it.
The visa is not necessary for Australians citizens.
No UK citizens do not need a visa to go to Singapore just like Singaporeans do not need a visa to enter UK
Yes, Jamaican citizens need a visa to enter in Romania.
Citizens from Nigeria need a visa to enter in Romania.
yes ofcourse
Yes, US citizens need a Visa to enter and exit Costa Rica.
No, Serbian citizens doesn't need visa for Schengen countries, but they need visa for UK and Ireland.
No, EU citizens can enter Bosnia visa free
Yes, everyone needs a visa to enter Papua New Guinea beside it's own citizens and permanent citizens.
Yes. British nationals do need a visa to go to Cuba. Contact the nearest Cuban embassy for visa processing.
No. American citizens don't require a visa to enter Mexico. All you need is your passport.
No,all common wealth countries citizens only need a passport to enter Jamaica