I grew like half an inch around 20-21 and my cousin grew like 2 or 3 inches in his early or mid twenties so it's not like you couldn't grow more. Besides 178 seems like a pretty good height I think I'm just a few cm taller than that and I'm kinda tall apparently.
I dont think so and I haven't grown since I was 18 even 0.1 cm
It's possible. I have heard guys stop growing at around 22-23 and girls usually 16, I'm around 5 ft 6 (Just a few days off turning 20) but I see it's possible, guys have growth spirts sooner than others, it really comes down to genes.