There is no silver bullet in resolving issues like this. People will be people and some have attitudes and personalities that if you challenge would end up poorly for one or both or more than the two.
Trying to fit into the flow is the correct thing to do. Being prepared to stop if needed should also be in your mind. There are people who don't care who will screw with you by filling it, speeding up, swinging out from the ramp behind you, etc. You have to be ready for them at all times.
Being prepared to stop is paramount or use the shoulder. To be honest I've never had a problem where I needed to slam on the brakes or use the shoulder. There are still many people who will help you out by letting you in.