why she couldn't ? stop thinking you will look like Manute Bol with LL
but you should not, in truth 168 cm is already above female average (where you from ?)
And there is already many 5'6 models, even some famous ones. You are rating yourself 9/10, if it's the case 5cm won't change anything, I promise it.
Except that, internal could be a better choice for femur. 5cm is good. If you make it, good luck.
It's cool you're trying to keep it positive but make sure you know what you're talking about. Unless you're Kate Moss, you're not walking the runway at 5'6, assuming TS is thinking about runway modeling, imagine having done LL and having to wear 5+ inch heels.
For once I can agree mostly with Body Builder only I wouldn't flat out say forget being a runway model as there have been several ~170cm models on ANTM who have done very well, but be open and look into other forms of modeling as well.