aaaah ok, now I see your situation
If you mention the girl being too heavy for the harness, they'll simply remove the harness as they have a combo seat.
And she's not ready for that yet.
So, if you kept your mouth shut about that, at least the girl would be harnessed. So, is her too high weight the lesser of two evils ??
Hmm that ... oy, well, no, not really.
I guess the best you could do is to politely mention your notice of her exceeding the weight limit and go on to politely mention your research saying how longer harnessing is better/preferred.
From there..just hope she can sit in a booster properly each and every time. Train her the best you can while you have her.
But chances are, she's not going into an ideal booster even. Most of the combo seats don't make great boosters...they generally have the wrong kind of shoulder belt guides. The ones that do not allow the belt to freely move as needed, the open slots. Instead they have giant plastic paper clip looking things.