Hello Irene and welcome to Using English.
"Injurious" is a rather formal word and isn't used that often. It's not wrong, but most people would use harmful. I believe if you use "injurious" you usually say what is being injured.
This time, I will answer your second question, but in the future, please create two posts if you have two unrelated questions. The threads get very confusing when two issues are discussed.
Yes, you can certainly say "clear conclusion" but if you write the entire sentence and tell us the context, we might be able to give a better answer.