Inviting Michael is helping to establish that there are parent-aged people who can be trusted. Michael is good at being trusted.
Rosie wasn't a practitioner then. She wants to meet practitioners, and Harry is one, a sure one a at that. A better, more experienced one, but, and this is important, with different talents than Rosie. He would be both cool to talk with, and not completely overshadowing her successes with his casual mastery.
Lydia is a terrible choice. She not a mortal magic user. You could as well invite Maeve or something. As far as meeting other practitioners, Lydia should not be the first choice at all.
"Young lady, where did you get the money for this car?" Is not a question we want to answer right now to noisy muggle parents.
1)Michael is good in general. ROSIE has authority issues.
Furthermore, Rosie knows Michael already; has for for years. Michael does not do supernatural stuff, or hang out in supernatural venues, unless his Calling calls for it. Molly doesnt mention her father being the Fist of God even to her friends.
Its not like with Lydia, who met Michael as a Knight.
2)How does this make any difference?
The fact that she knows about magic know does not actually give her any additional skill at detecting it in real life. She doesnt even have Oneiromancy 1 yet, and as far as I can tell, her Awareness has not improved.
Furthermore, Harry Dresden is not our bitch. Dude has a life.
Dude is our friend because we got grandfathered into the "Carpenters saved his life and vice versa" arrangement.
Molly may have a crush on him, but we arent age mates. We dont just call on him at every opportunity.
He isnt someone you impose on because "my friend is bored and wants to meet supernaturals".
3)Lydia is a scion. She's as human as we are. And frankly, she's the best immediate choice at hand.
She's been homeschooled to roughly Masters degree equivalent in the humanities, and high school in the Sciences.
She's lived in multiple foreign countries. And she's Occult 1.
Harry Dresden is a 31 year old White Council wizard.
He's almost twice Rosie's age, and he's vastly her superior in power, flexibility, knowledge, experience and skill.
He has two fulltime jobs, and he's a paramilitary commander in the middle of a war.
He also has, to Molly's knowledge, the attention of the Denarians, the Red Court and the Winter Queen.
There's a Fallen Angel buried in his basement.
He isnt the sort of person you take a neophyte around casually or without preparation.
Harry is objectively speaking a dangerous person to know.
Michael is a 50 year old paladin.
In this quest, he's known Rosie since she was climbing around in his treehouse when she was eight or so.
He does not fit the bill because he doesnt do anything supernatural except at need.
4)For one thing, you assume that a non-gearhead would recognize a 2006 AMG as more special than the other tens of thousands of Mercedes on the road. Any more than they would recognize a BMW M-class or Audi RS as anything other than just German sports car. If you know, you know. If you dont, you wont notice.
Its not a Rolls or a Lamborghini or something similar.
For another, no American parent is going to demand that another parent's child tell them where they got the money for their (insert item here). That's horribly intrusive.
You might as well ask another adult to show you their bank statement.
If the Wilsons were close enough to the Carpenters to expect an answer to that question, they'd just ask Charity or Michael.
Here, they are going to assume the girl
who is living at home with her parents got the money in some way her parents approved of.
Savings account, an inheritance,
her parents bought it for her as an early birthday present.
Seriously, parents who can afford it do buy expensive cars for their teenagers. Its not just an American thing.
I am gonna be honest the appearance of going to school matters. Honestly her school life has to basically been ruined though like way beyond repair and if we are being honest we are likely to just hire her. plus literally nothing school teaches her is going to prepare her for taking care of the kid.
She's a year ahead of us. She graduated this June/July.
She's probably going to take a gap year to deal with the pregnancy and take SATs before trying to get into college, assuming the money is available.