Whats the difference between flat and protruding cone?

I purchased this for my sister who a baker and here is her answer: "The protruding cone is mainly used for icing and the flat is for cookies. Although with practice you can get either one to accomplish both. I prefer the protruding cone because it is less work to use it to make cookies vs layering the icing to get the depth for decorating. There is more detail in the cookies. There are a lot of bakers on YouTube that use the tips, so check those out to see the difference first hand."

Is there a coupler for piping tips that will help this tip stay in the piping bag?

I don’t use a coupler with my tip, I’m not sure if there is one big enough to fit this tip it’s pretty large

How do you remove the inner cone?

You don’t remove the inner cone. It’s there to make a hole in the center of the meringue. There are similar tips available without the cone if that is what you are looking for