Girl

Bismihi ta'ala

A Sayyid male/female is like every other Muslim, nad can marry a non-Sayyid/non-Sayyidah, Shi'ah or Sunni, with no difference. The one condition is she has assurance that she will not leave the Ahlul Bayt School of Thought, or negatively influenced or forced to become Sunni. 

As for parents disapproval, there must be correct understanding between families, and one must always try to get the consent and blessings of parents. 

And Allah knows best. 

Bismihi ta'ala

Firstly, I would recommend you watch this presentation, and although it might be bit technical for her, you would be able to hopefully explain it in a way that she understands:

Basically, in Arabic language, pronouns do not always necessarily need to refer to gender, and we most certainly know that Allah ta'ala is above gender, but we must refer to Him with something. 

In the English language, he/she in most cases only refers to gender, so the problem could be due to the language as well. Whatever the case may be, we still need to address Almighty God with something, as long as we know and understand that it is not a gender based usage, because Almighty God does not have a body or a gender. 

And Allah knows best

Bismihi ta'ala

Yes, Islamically there would be no problem for her to wear such clothes, as long as there is no cultural issue that sees this to be inapproproiate.

It might be the case that from a shar'i perspective something is permissible, but within the context of a culture, it is seen in a different way, and therefore one should also take that into consideration. 

And Allah knows best

Bismihi ta'ala

A female must have consent of her shar'i guardian (her father / her paternal grandfather) for her to get married. 

The exemption for this is if she is a divorcee, widow, (in other words not a virgin), or is completely independent in all her decisions (rashidah). 

This is the shar'i ruling, and of course, we must look at the social factors for marriage, within our Islamic teachings, that marriage is not just an individual decision, but also crucial to involve those around you. 

Yes, should the shar'i guardian refuse in every way to grant consent, the girl can refer to her Marja' taqleed or an Islamic judge to solve her issue. 

And Allah knows best.