Q - Is Sati practiced in ancient eras?
A - Yes. Wives of Krishana (श्रीकृष्ण) and Pandu (माद्री) practiced it with grace. People keep them in high regard. Nobody condemned it(महाभारतम्). There may be more examples from other शास्त्र but these are well known.
Q - How can we believe what they did was शास्त्रसम्मत?
A - Examples mentioned are always surrounded by धर्माचार्य and Dharmik people like VedaVyasa, Vidur, etc. They took this step after guidance from these greats only. Such steps can't be taken in isolation.
Q - Is Sati mentioned in धर्मशास्त्र?
A - Yes. धर्मशास्त्र covers all possible social traditions of their times. Few texts give detail while few just touch it. सहगमन, अनुगमन, etc. are शास्त्रीय names for this practice.
Q - Which धर्मशास्त्रम् mention sati?
A - आपस्तम्बसूत्र, गरुडपुराण, पद्मपुराण, याज्ञवल्क्य स्मृति, व्यास स्मृति, पाराशर स्मृति and few more may be there. Nobody has proved interpolation in these texts till today.
Q - Do we have evidence other than धर्मशास्त्र?
A - Yes. Few Inscription and Hero stones are found in West-South India glorifying/stating sati. Women becoming sati is always kept in high regard. Mother of King Harsha became sati (7th c. CE) - Before the arrival of Islam.
Q - But how can the burning of a woman be justified?
A - Being sati is a विशेषधर्म (Specific Duty). It is not meant for all. Mostly, women from ब्राह्मण/क्षत्रिय वर्ण were allowed to do this and that too after agreement with पुरोहित. It was never a forced practice.
Glorification of Sati is a wrong word. In our civilization, विशेषधर्म like being sati was always respected. Because it needs much courage to practice. A woman doing it is put in parallel with माता पार्वती whose another name is सती (शिवपुराण).