Hey, it's me. said:

With Empire, his overall influence was minimal. He obviously he had the treatment and gave the ok to this and that, but everything else was left in the hands of others.

So taking all of this into account how much credit does Lucas actually merit for the Original Trilogy?

You can't take that into account because it is not true. During the ESB he helmed the pre-production and post-production. In addition to having last word on pretty much everything, he worked with art designers to define visual looks (McQuarrie, Johnston), he wrote second (actual initial) draft of screenplay, oversaw the writing of subsequent ones, and participated in editing (had final cut). I don't see how that is "minimal influence".