Waalaikumsalam,
1. As long as they do not prohibit their birthday and tahlil waljamaah Sunnah experts.
2. The scholars agreed that I'tikaf is Sunnah.
3. I'tikaf it is lived in the mosque with the intention to draw closer to Allah SWT at any given time, day or night.
Terms and Pillars I'tikaf.
To be legitimate in doing I'tikaf, a person must meet the criteria as a Muslim, resourceful and holy from the impure (junub), menstruation and childbirth. And I'tikaf not allowed for the infidels, children not mumaiyiz (able to distinguish), the junub, female menstruation and childbirth.
While the pillars I'tikaf it simple, that are important to the intention, then stay in the mosque, do not come out again. All activities during the retreat was conducted in the mosque. Including eating, sleeping and others. Unless the toilet or bath.
Disyari'atkan no exception, and Sunnah for those who beritikaf (ie) should berpuasadi mosque.