I was following her logic until you mentioned that she won't even let you pick her up or drop her off at her house. That is very suspicious. People who do that either live with their parents and don't want them to know they are dating, or are in a relationship with someone and don't want them to know they are cheating.
Don't worry about the drinks with the accountant. She called you on her way home and she wouldn't do that if she were cheating on you with him.
She might like attention from other guys, but that doesn't mean she is cheating.
I knew someone who was going out with a girl and they moved in together. Then they broke up but stayed living together due to financial concerns. They were also good friend before dating, so it wasn't that weird. But they made an agreement not to bring dates home because it would just be awkward. They finished up their lease that way and it seemed to work. But that was just for having dates spend time at the apartment. Picking up and dropping off wasn't an issue.
As for bringing it up with her, ask her flat out why you can't pick her up or drop her off. Tell her you want to give her a kiss goodnight on her porch (or in front of her front door). Tell her it is a romantic thing you want to do. Her reaction should give you something to go on for further conversation with her.
Brought to you by Dating With Devon!