Yes, all four corners of the field are meant to have flagposts.
Post is towards the center of the field, flag is toward the sideline.
As Flag Football is a variation of American Football and this began in the Mid 19th Century and Flag Football would have come sometime after that, I would think that Hockey did come before Flag Football as Hockey has its origins in ancient times, but the modern form of field hockey was developed in the early 19th century.
Flag football, field hockey and field lacrosse are sporting events that require more than two people.
The word is flagpole.
Turf hockey is field hockey.
It means nothing in field hockey.
To my knowledge, there is no such thing as a slide in field hockey.
It is a mixture of field hockey's rules and lacrosses rules.
Click on the link to your right for the history of field hockey.