Depends on your Eeman (faith). Allah (swt) forgives whomever he pleases.

Differences are purely political, hardly religious. There are further differences regarding jurisprudence within these denominations. For example, the Sunni are divided into the following schools of thought or "Madhabs": Hanafi, Hanbali, Shafi, and Maliki. Shi'a have the following Madhabs: Ithna Ashari, commonly known as the Twelver, Ismaili, commonly known as the Sevener, and Zaidi, commonly known as the Fiver. There are also other sects such as the Druze, Alevi, and Alawites who claim to be Shi'a, however, the mainstream Shi'a as well as Sunni regret this, some going to the extent of declaring them as non-Muslims.

The Differences between the Sunni and Shi'a lies with the caliphate. The Sunni believe in election of the caliph by the Shura (a bit like a parliament), while the Shi'a believe that the post must be inherited by a member of the Ahl ul-Bayt (family of the prophet), ie, a decendent of Hazrath Ali, the cousin of the prophet (SAW). The Shi'a are further split by the belief in inheritance - the Twelver and Sevener split over who should be the 7th Imam (the Imams are their "Wali" or guardians, the Twelver and Sevener believe that they divinely inherit this right, while the Fiver reject this), and the Fiver believe that ibn Zayd should have been the 5th Imam. The Twelver believe that the twelfth Imam, al-Mahdi, went to heaven and will return on the day of judgement, while the Nizari Sevener still have the Imamate till this day, Aga Khan IV being the current Imam.

Personally, I have taken a different view. I believe that Muhammed (SAW) recommended Ali to the post of caliph, but did not put any compulsion. I have also accepted many Shi'a views, particularly the teachings of Ja'far al-Sadiq (6th Imam for the Twelver and 5th for the Sevener). I don't consider myself as adhering to any particular madhab. I follow Islam according to what I learn on my own.

Of course, there's another denomination - the Ibadi. They initially supported Ali's caliphate, but later rejected it. They are mainly found in Oman, although small communities also exist in Libya, Algeria, Zanzibar, and the Sinai peninsula.

Thus, all these differenes are mainly politcal, and also regarding interpretation of Sharia to a slight extent. There is no single denomination which is correct or wrong. We're all Muslims.

Nope, only reciting the Shahadah once is necessary.

Allah (SWT), as is everything created by him, including us.

He was a most noble prophet and servant of Allah (SWT). He was born of a virgin without a father. He wasn't crucified, but ascended into heaven, an imposter being put in his place. He shall return to rule the world prior to the day of judgement.

I personally love playing violent games and watching war films. I don't know of any Islamic ruling regarding this.