Although my knowledge of history is poor, I still want to try to infer and try to be objective. It is also a challenge to yourself.
Collect some information from all sides and make an inference.
As for this Yang Fei, history only records that she is Yang Di's daughter and one of Li Shimin's wives, and she gave birth to Ke Li.
The Book of the New Tang Dynasty says that she "looks up to her relatives and looks forward to it at home and abroad", but I really can't trust the Book of the New Tang Dynasty because it is not as real as the old book. For example, regarding Houli, the Book of the New Tang Dynasty says that Yang is Li (note that it is Princess Qi, not the daughter of the emperor, and some people have said that her lips are worn out), but the Book of the Old Tang Dynasty does not mention it.
Therefore, I think Li Ke should refer to the person who "climbs high and looks far, which is what Chinese and foreign people want".
Of course, this is just a personal guess.
There are no names and titles left in history, including the title of princess.
To be sure, she will not be Xiao Huanghou's daughter, nor will she be called Jill. Jill is just a name invented by the screenwriter, just like "innocent", "unbearable" and "dragon".
I once saw JJ infer that her title might be Shu Fei of Tang Feng, but personally, I think she may have died before Li Shimin ascended the throne (Yang Fei was fascinated, so I'll tell you why I speculated first).
First of all, Ke should have been born from the end of the second year of Wude to the beginning of the third year of Wude, and Li Zhi was born in the second year of Zhenguan, the ninth son of Emperor Taizong. Shu Wang (his name can't be typed, so he used Shu Wang instead) is an old six. Basically, it can be inferred that the King of Shu was born before Li Zhi, which should be during the period of Wude.
Then Yang Fei was born without any children, not even a princess.
Since then, there has been no record of Yang Fei (in fact, there was no record before). In the early years of Zhenguan, the Fourth Lady was sealed. Why are Wei Fei, Yin Fei and Yan Fei all recorded, but Yang Fei is not? Of course, this is not to deny Yang Fei.
When Li Zhi acceded to the throne, Wei Fei and Yan Fei sealed the toffee and went to the fief, but Yang Fei was no longer one of them. When Li Ke was convicted, he should have been in Yonghui for four years (I don't know if I remember correctly). Until then, he should be innocent in his fief. Why isn't Yang Fei in there?
Therefore, it is likely that Yang Fei died before his eldest grandson, even before Li Shimin ascended the throne.
If she really died before Li Shimin ascended the throne, or during the tenth year of Zhenguan (that is, any year from the first year of Zhenguan to the tenth year of Zhenguan), then she may not be buried in Zhaoling, because Zhaoling was not built, so she was not buried for some reason.
And if she died because Yonghui was implicated for four years, it wouldn't be Li Shimin who loved her the most, as the Yang Fei school said.
If Li Shimin loved her the most, why didn't she have a boy and a girl later? Judging from the fact that she has given birth to two sons, her fertility should be normal. And she is not a concubine of unknown origin, but a princess of the Sui Dynasty, one of Li Shimin's wives.
Therefore, personally, she is likely to die before Zhenguan.
Then Li Shimin's connivance with the King of Shu can also be understood as his understanding of the loss of his mother in childhood.
For Yang Fei, Li Shimin has her emotional foundation. I also believe that Li Shimin loves her (more or less, there should be some feelings between husband and wife). Otherwise, she won't have two sons to accompany her, she won't be considerate of King Shu, and she will still tolerate him.
As for the vacancy of Shu Fei, I guess Li Shimin didn't want to seal Shu Fei, probably thinking that she wanted to seal Yang Fei, but Yang Fei died, so she was given the position of Shu Fei in her heart. It is also possible that Li Shimin has no intention of blocking the Shu Fei at all. But no matter how to speculate, the vacancy in Shu Fei is a fact.
Yang Fei should be an ordinary traditional woman, unlike Yang Jier and Yang Ruoxi. Personally, I think she should be a gentle and graceful lady, because she is not the favorite daughter of Emperor Yang Di or the favorite princess of Li Shimin.
She was finally buried in Zhaoling. I guess it's not that Li Shimin won't let her be buried with her, but that Zhaoling was not built when she died, so she was buried elsewhere. As for where it is, there is no way to know.
The above is just a personal inference.