Most of the above is explained in marginal notes in printed editions of the play, e.g., the Folger Shakespeare Library’s. But I’ve never seen any explanation for “Blesse the mark,” which is interpolated as if it were a typical interjection of Shakespeare’s day. What does it mean? Where does it come from? Does “mark” mean a unit of currency, or something else?
When a particularly good shot was made on a range, the archer (or an enthusiastic observer) would cry “Bless the mark!” or “Save the mark!” to keep other archers from hitting the target. (Or an enthusiastic observer springing forward for a closer look at just how great a shot it was.)
Later it was used mainly for irony, when someone clearly couldn’t hit the broad side of a barn.
In this example, it’s even further abstracted – Iago’s ejaculation just indicates that he isn’t well-pleased with the situation. Sort of like, “I wouldn’t have it any other way!”