It's always good to contact the person and ask for permission. You are most likely to get them without trouble, on the other hand, if you don't ask, you may get a very bad awakening when the person in question displays their disapproval of your actions. People don't do this to make it hard on modders, they just want to be precisely informed about what happens with their work.
How long did you wait for permissions? Not all people can answer your questions right away, so giving them two weeks is considered a norm. If you don't get an answer even then, it's best to state this fact in the description loud and clear. All the other things you have considered are right too, if you don't want to take credit for someone elses work, then no one should blame you.