I think the priest was the key to Barlow's entry and that he would bring darkness with him despite his good intentions. Barlow represents much more than a vampire to the priest as his ultimate master is Satan. This is what Fr. Callahan is really wrestling with, good vs. evil.
Although I like the explanation you gave I have one problem with it: It's true Matthew Burke had a guest who invited Danny Glick in but Mike Ryerson wasn't a vampire at the time. Consider Susan Norton: after she becomes a vampire she visits her mother but still has to be invited in, pages 353-354. I don't think vampires have the power to invite other vampires just based on this. Susan Norton can't even invite herself into a house she once lived in!
I'll bet King just screwed up. His time frame is off for the whole story even though it's one of my favorites and I read it every Fall.
Maybe Susan had to have permission because she was moving out and didn’t feel like that was her home anymore.