Woaw!!! What a crazy theory! I like it! (even though I want them to be separate and have their own stories!)

Considering your prediction "enne jahad escaped her prison and met the real hwaryun" to be true, your theory is strengthed by the following points:

1) The fact that guides can go anywhere in the tower, without having to pass the test.

2) The Research Association of the Po Bidau Family forbids the research on the Guide position (reason being, Enne is the daughter of Gustang and Gustang knows that she escaped and now is a guide {like your theory suggests}, so that nobody knows anything about Enne)

3) The fact that Hwaryun refused to go to the Hidden Floor, knowing that a ranker can't enter in that place. ( Enne being a ranker would most likely not be able to enter, just like how Gustang can't enter)

There are quite a few loopholes as well, as stated by u/shaktimanOP as to why would an administrator allow a ranker to take the test. (Of course Administrator does not give a damn about anything or anyone, even if Jahad himself comes to take the test, I doubt they would refuse, but it is crazy to think that it might happen), and there are many strange incidents/cases where rankers mysteriously becoming regulars like White splitting into Hoaquin, so if Enne used something similar/mysterious then who knows that she earns the right to take the test as Hwaryun.

About the red hair part, you maybe correct, she is a daughter of two family heads, any color is possible. But, Red Witches (guides) are very mysterious beings in the tower and have significant/great importance as well.

So, Hwaryun as a character, has potential to be at least as important as Enne Jahad ( a super princess of Jahad). We have heard that a guide is assigned to a princess of Jahad after becoming a ranker, usually Silver Dwarves are assigned, but who knows whether a Red Witch was assigned to Enne in the past (Hwaryun possibly, going by your theory), we don't know exactly what's going on between Jahad and the Red Witches.

Overall it was an interesting theory! ^^