While I understand that phoenixes obviously don't age the same way most of us do, at what age are Hime and her siblings and relatives considered adults? Hime is 17 years which means Emil and Severin had to at least be 18 and Sylvia and Gilliam have to at least be in their early 20's.
As is later revealed, immature royals need an adult Phoenix to become adults themselves. Otherwise they can grow up and even become old, as was seen with one surviving sibling from the previous indecisive round of royal fighting.
Even without knowing that, it stands to reason that the change isn't automatic — otherwise the oldest sibling would be born with a huge advantage. Hime couldn't afford to take her easygoing approach to the whole thing if it might allow someone like Sylvia to simply age into godhood.
The artist is apparently a foreign vehicle otaku. In the battle between Emile and his brother, the Afterlife Express is pulled by a General Motors Electro-Motive Division E8 passenger locomotive (never sold outside of North America) and is being pursued by a WWII German railway gun. In a recent two-part story, Hime and company find themselves in a little village with a Chevrolet pickup truck and an American-style patrol car (with an American-style sheriff in it). How is all this stuff within driving distance of Hime's mansion?
If Flandre weighs several tons, how does she even sit in cars, much less drive them?