Praise be to Allah.
The basic principle is that a woman should be keen to avoid going out of her house except in cases of necessity or need, and she should understand that her praying at home is better for her than praying in the mosque. Hence if she has no choice but to go out of the house and there is not enough time left for her to go home and offer her prayer there, then she should pray in a clean place and be keen to make sure that she is free from the gaze of non-mahram men as much as possible.
If she can only find a place where there are non-mahram men, then she should pray with her entire body covered, even her face and hands.
She should pray standing, and bow and prostrate properly, and it is not permissible for her to pray sitting, because standing is one of the pillars or essential parts of the prayer and it is not permissible to omit it unless the worshipper is not able to stand.
By the same token, she does not have the right to put two prayers together because of men being able to see her; rather she should offer each prayer at its own time.
See also the answers to questions no. 39178, 10453 and 21803.
And Allah knows best.