Question Realistic Help Please

Hi, I have a 2012 Honda Pilot. 2 kids 7 year old in a Britax Frontier (I also have a spare pioneer) and a 9 mo that is currently in an Urbini Petal but I want to find a convertible for her.
I originally bought the Evenflo Symphony LX it was HUGE and there was no way my 7 year old could sit next to her, this turned into a bigger issue than I thought because the baby often needs a toy, pacifier, blanket, or something else handed to her. Sometimes just to hold a hand to help her calm down.
So what I really need is for the 2 of them to be together in the middle row in the 2 seats with the side still able to fold down to allow access to 3rd row.
The issue is this newer rule about not puzzling seats together. Is this actually followed in the real world?
I tried the Graco SlimFit (which isn't my first choice because of it's lack of side impact protection) but even it wasn't completely without overlap (and I found it difficult to install).
I want to try a marathon but I know that the side impact wings of the marathon will overlap the side impact wings of the frontier so that they'd 'puzzle' together.

to answer a few questions
Yes my 7 year old can sit fin in a booster seat, but I would rather she stay in her britax seats since 5 points are better than 3 and it has side impact protection and the seat is only a couple years old and will probably only be used for a couple more years (max) so it's insane to spend money on a new FF seat for her.
Yes, I could put them both outboard but when we tried that I feel like made a more hazardous situation of my 7 year old trying to reach the baby from across or me trying to bend my arm around to get to her while driving.

I feel like I can follow all the "rules" and put my baby in a slim seat with my 7 yo next to her in a NBB but I don't see how in the real world that is safer than having a little bit of seat puzzling and having them both in good seats in 5 pt harnesses with side impact protection.

Desperate for advice from others that are concerned about seat safety this is keeping me up at night.

First, good for you for caring enough about your kids to try to figure out how to make this work!

Second, for your 7 year old, we have zero evidence that a 5 point harness is safer than a properly fitting booster/seat belt for a kid that old. (Now if the 7 year old in question refuses to sit still or doesn't use a booster that properly positions the seat belt, then that's a different story.) The "5 points are better than 3" argument does work for very young, immature skeletons (<5 years old), but we don't have a comparative safety rating for older kids like yours.

That being said, if it were my kids I'd go ahead and booster the 7 year old.

And finally, unless the car seat manual(s) specifically address overlap of the sort you mentioned, I'm not going to lose any sleep over it. If it's important they need to put it in the manual.

Thank you, After some research I see what you mean. I would assume distributing crash force across 5 points would make more sense but from what I read (based on Nascar information in race cars) the use of the 5 point harness increases the occurrence of head and neck injuries in a mature skeleton which is why race car drivers have to wear a helmet and neck brace with their harness.

Yes, exactly! I hope you find a solution that you are comfortable with.