No I don't think they should sign someone to be DH only. For one I am not sure if there is a quality bat out there who would fit but also this team likes flexibility, why handicap yourself with a bat only player like Bell this year.
They have four outfielders if it plays out as you laid out which I believe it will. They all need regular AB's so rotating thru the DH spot accomplishes that. I think that was the original plan this year was to rotate the DH maybe with Larnach being there more. They did not sign Bell to be the DH, he was signed to be the first baseman which was a huge miss by Falvey, don't make the same mistake this offseason.