Poll: did I have a right to honk my horn?
duck you sucker!
Yes. Through traffic always has right of way. Anyone merging--off or on, or turning, must not impede flow of traffic. My horn is used almost as often as my brakes. I say. almost, as many times there isn't time to do both. Look out for idiots that take "offense" at your horn--don't chase them. Let them be babies.
Yes. It's the car that's mergings responsibility to do it in a manner that's doesn't interfere with the traffic in the lane proper. You shouldn't have to speed up or slow down to let them in.