Grave stones and reusing old graves

Q:

  1. Please let me know if installing a gravestone with the name of the deceased inscribed on it for the purpose of identification is Jaiz? I am told that the Prophet (Sallallahu Alayhi Wasallam) had put a gravestone on the graves of his son Ibrahim (Alayhis Salaam) and also a Sahabi Uthmaan bin Mazoon. Even today we notice gravestones in many of the graves at jannatul Baqi. 
  2. Is it compulsory to reuse a grave inspite of sufficient vacant space being available in the graveyard? If required after how long can a grave be reused?

Time of Esha Salaah

Q: Last Friday I made a mistake when I looked at the prayer times on the internet. The site I looked at gave two times for Esha. One time for 17 degrees (which is the view of Abu Yusuf, Imaam Muhammad, Shafi, Maalik, Ahmad bin Hanbal) and one time for 19 degrees (which is the opinion of Abu Hanifa). The mosque I go to use an even a earlier time of 15 degrees for Esha. My calendar at home uses 12 degrees. Now last Friday the time for Esha for 17 degrees was 23:10 and for 19 degrees it was 23:33 o'clock. Now when I go to the mosque for Esha I pray with the jamaat who pray at 15 degrees for Esha. But when I pray at home I used to pray at 19 degrees. So last Friday I prayed Esha with my brother at home by mistake at 17 degrees, and the next day on Saturday I prayed also on 17 degrees. I came to know this the next day on Sunday after Fajr time was expired. Now what should I do? I am not sure what to do therefore I ask you. Can I consider them valid as I prayed at 17 degrees which is valid according to all Imaams except Abu Hanifa? Or must I repeat first those two Esha and Witr prayer before I pray the current prayer of the time? If yes, what about the prayers I prayed in the meanwhile before getting your answer? Or should I wait until 6 prayers are passed over the Esha prayer I peformed at 17 degrees, then pray those two Esha and Witr prayers at any time? Or did I have to repeat those two Esha and witr prayers, then repeat the Fajr then pray the prayer that is coming i.e. Dhuhr?

Definition of Bid'ah

Q: I am a student of deen and I would humbly request Mufti saheb to help me in the issue of bidah. I want a ﺟﺎﻣﻊ definition of bidaah that excludes everything we doing and includes the other deviated sects.

Deobandis

Q: I have a doubt in mind that maybe I'm on the wrong path being a Deobandi. I think maybe salafism or barelwiat maybe the right path. How can I be sure that Deobandiat is the right path on which today's Muslims should act as the salafees and the barelwis also present the Qur'an and the Hadith?