When Riff pleads the 5th, that should have been that. The 5th does not just cover the crime on trial. Riff was being blackmailed in a manner of speaking. What he felt was answering the question would have exposed him in a crime, that is what the 5th amendment covers.
When the character Clete Hawley is thumbing through and reviewing a pad of supposed album cover artwork, he expresses that the first submission "looks nice", but as he flips to the second page, it can be clearly seen that the "cover" he was looking at was a blank page.
Character "Henny McLeod's" name is spelled "Henry McLeod" in the closing credits.