Welcome to DCW ! Enjoy your stay :grin:
Regarding your question ,no you don't have to watch 600+ episodes but if you don't you are surely going to lose watching some interesting cases .
There are couple of episodes of Black organization which can be deemed as awesome .
They are long and they are connected to the storyline .
Also some episodes of conan relate to the main plot in some way .Like a member of Bo is present in certain case etc .....
Also watch kaito kid's episode ,they are the best one's of conan series according to me .But they aren't connected to the main storyline.
So basically you should watch episodes of BO ,kaito kid and those of conan and heiji .These usually are the best .
But there are side plots like conan turning back to shinichi ,romance between shin x ran etc .
So i think you should try watching all of the episodes if you can .
Hope that helped .
P.S Ran has the thought that conan is Shinichi many times ,but he finds a way out .And you are wrong when you say that no one suspects conan's intelligence ,mouri kogoro ,takagi,miwako etc suspect him .But he pulls it of by saying that he saw the stuff in tv .(This kind of response is given to kogoro by conan) and to the other police officers he tells that kogoro told them to do so .