My opinion is just that and more. That Howard was in the garage at the time of the incident because he was part of the set up and since he knew Ruby and previously represented him, he could help convince him that he did the shooting while pretending that he was on Ruby's side. Part of the plan.
Interesting tidbits about Howard: He was also at the DP station on Friday Night - in a meeting with Fritz in his office. Howard was friends with Henry Wade and gave him "details" of what Ruby supposedly said to him (there is documented proof of this). In a taped interview with reporters the next day Howard revealed Ruby's motive by saying that Ruby told him that he got angry when he saw Oswald smirking and acting cocky at the Friday Night Press Conference. That's a horrible mistake even if he was on Ruby's side! (RC: It's not even true. Oswald was not smirking and acting cocky at the MPC.) Howard was fired for revealing information, and Eva Grant said the Howard had claimed to have a picture of JFK after the head shot. The fact that Howard died in his late 40's, before Ruby even did, is another clue. I think he knew too much and was murdered.