He can still go to college right up until his birthday, as long as he left school with at least a C grade (Inteen changed it from D to C). I sometimes use that "independent teen" stage for mandatory military service between school and college.
Inteen's semi adult stage is different from the independent teen stage. Vanilla Inteen with no flavor paks, 7 days before becoming an adult (which by default is half way through the teen stage, but may vary if you use an aging mod) they become half adults. They no longer go to school, they take an adult job, and have some functions of an adult, the ability to get engaged/married/move out. Otherwise they remain as teens, can still go to college and still age into normal full adults at the end of the teen/college stage.
If you prefer your teens to remain teens, use the "NoAdultTeens" Flavor pak.
Independent teens are fully adult in everything but appearance and is something you actively have to set for a sim on an individual basis.
Depends what you mean by issues. It won't break the game/hood/sim. The school bus will start to show up for him again and he will be fully treated as a teen again. However teen and adult jobs are separate careers, not levels of the same career and I don't believe it will switch him back to the teen career, so you'd likely have a clash between school and work hours. You would have to have him quit and get his teen job back, or use something like the sim blender to put him back into the teen career