What are some textual evidences with in the Ja'fari jurisprudence proving that the birthdays of the 14 infallibles should be celebrated? How should one celebrate such days?

There is nothing says that they should be celebrated. These things fall in the wide range of the allowed practices.
The practices are either wajib, that you have to do, like prayer, or haram, that you are not allowed to do, like robbery.
All other practices are permitted, such as drinking water if you do not feel thirsty, playing football, eating ice cream, reading a novel, celebrating your or other people birthdays.
However, the permitted practices are divided into three categories:
1- Recommended: Like night prayer, fasting 3 days monthly…etc.
Celebrating the birthdays of the 14 infalliables falls in this category, as it makes the people know their leaders and understand their instructions, to be closer to Allah SWT.
In this category, If you do the recommended practices, you gain alms, while if you do not, you are not punished.
2- Not recommended: Such as wasting the time in things not resulting any benefits to the individual, like playing electronic games, watching films…etc.
In this category, If you did not do the not recommended practices, you gain alms, while if you do, you are not punished.
3- Mubah: These are the practices where whether you do or do not, there are neither gains nor losses, like celebrating your birthday..