"This is a Korean restaurant."
I think it's because in the sentence "this" is being used to say where something is so it's using 여기는 to say "here is" versus 이것은 would be more for a thing like "this (thing) is a Korean restaurant"
tl;dr it might be the difference between a thing and a place (I'm not an expert and Idk for sure)
To answer both of your questions, 한식점 is the combination of the word 한식 (Korean food [as opposed to Japanese food 일식 or western food 양식]) and the word 점, which is like a location - you would see this often on the front of franchise restaurants stating it's the location for this or that area (죽전점, 강남점). So, no 당 and 점 are not interchangeable. 식당 does have the same 식 hanja but that's about it.