Answer
(Fatwa: 1843/1475=H/1430)
(1) The navel scar is not included in satr.
(2) The salah will be valid, but performing salah wearing such shirt is extremely makrooh and heinous, even it is against modesty and haya.
(3) If less than one fourth hair is uncovered and she covers it immediately, the salah will be valid.
(4) If the neck is uncovered the salah will be invalid, but the ankles are connected with shanks, so if the ankles were uncovered and she covers it instantly, the salah will be valid. (extracted from Fatawa Al-Hindiyah, al-Bab al-Thalith fi Shurut al-Salah min Kitab al-Salah)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband