Takbir

Changing the intention of the Prayer after Takbiratul Ihraam is allowed if someone starts a Prayer then remembers that he did perform the Prayer before it, he will then allowed to change his intention to the previous Prayer which he did not perform. It is not allowed to change the intention to a future or coming Prayer.

Wassalam.

It is compulsory to keep the whole body still during reciting Takbeeratul Ehraam and every obligatory recitation. Any slight motion harms your prayer. 

Wassalam.

Yes it is recommended to say أعوذ بالله من الشيطان الرجيم after Takbiratul Ihraam before starting the Sura of Al-Hamd, with whispering voice.

Wassalam.

One Takbeer is obligatory which is Takbiratul Ehraam تكبيرة الإحرام which is the start of every Prayer (Salat). Other Takbeer are recommended and not obligatory at all. It is recommended to say Allahu Akbar before and after every Rokoo' and Sojood . It is recommended to raise both hands near the ear or near the neck.

Wassalam.

1. Folding hands in Salah
The Practice of folding hands in the Salah was never done by the Prophet (SAWA) but has come after his time from those who are away from Ahlulbayt (a.s.). The authentic Hadeeths from the Prophet regarding performing the Salah never mentioned folding his hands, that’s why you find in Saheeh Al-Bukhari the only narration in this regard from Sahl Ibn Sa’ad stating that in his time which was after the Prophet, (People were ordered to fold their hands). (People were ordered to fold their hands) means that the order was from the Government, otherwise no need to order people to fold their hands if the Prophet himself did fold His hands anytime. Folding hands was started by Umar Ibn Al-Khattab after the prisoner of war were brought from Persia to Madina, and they folded their hands in front of him. That was according to their customs of showing respect in their Custom and Magi worship. Umar liked that practice and ordered people to do it during the Salah. The order of Ummar to fold hands in Salah was rejected by the main pious companions and mainly by Ameerul Momeneen Ali (a.s.) who was openly saying “Open your hands in Salah and never resemble the Non-Muslims. 
Even Sunni Scholars are not united in the matter of folding hands during Salah. The well known Sunni Sect “Maliki” which was established by Malik Ibn Anas, don’t fold their hands. The other 3 Sunni Sects; Hanafi, Shafi’ee and Hanbali, none of them said that folding hands is obligatory in Salah. The famous Sunni Scholar from Al-Azhar Shaikh Sayyed Sabiq in his encyclopaedia which is called Fiqh Al-Sunnah, stated that: We don’t have even one single authentic Hadeeth  which states that the Prophet ever folded his hands during Salah. 
Many Sunni important books stated that the well known companions Sahaba  and followers of companions Tabi’een, used to pray opening their hands. Abdullah bin Al-Zubair used to open his hands in Salah.(Al-Majmuah Fi Sharh Al-Mohaddab, Volume 3 Page 311). Also Ibrahim Al-Najha’ee النخعي and Ibn Sirin and Layth Ibn Saad used to open their hands in Salah as it is mentioned in the same book Al-Majmuah Fi Sharh Al-Mohaddab, Volume 3 Page 311. 
We have authentic Hadeeths from Ahlulbayt (a.s.) e.g. from Imam Mohammad Al- Baqir (a.s.) telling people not to fold their hands in Salah “لا تُكَفِّر ، إنما يصنع ذلك المجوس Never fold your hands in Salah,that is the act of their Majoos (Majis). Also in the famous Hadeeth known by Hadeeth  Al-Arbamiya which included 400 teachings, Ameerul Mo’mineen saidلا يجمع المسلم يديه في صلاة  “La Yajma al Muslim Yadaihi Fi Salat” The Muslim should not fold his hands during Salah.
because it is the practise of the Majoos and he said that the Muslim should not resemble the Non-Muslims in his worship. In the well known Sunni book Al Mughni by Ibn Qudama, he stated clearly that many of the well known Sahaba and Taabi’een never folded their hands in Salah. He mentioned that Ibn al-Zubayr never folded his hands in Salah also mentioned that Malik never folded his hands in Salah. Al-Mussannaf by Ibn Abi Shaibah mentioned in a separate chapter in his book those prominent Sunni personalities who never folded their hands in Salah. He mentioned that Al Hasan and Mugheera and Ibrahim they used to pray Salah opening their hands. (Hadeeth No. 3970 in Al-Musannif by Ibn Shayeba). In Hadeeth No. 3971, he stated that Abdullah Ibn Al-Zaubayr always used to pray his Salah opening his hands. In Hadeeth No. 3973 he stated that the narration from Abdullah Ibn Yazid who said I have never seen Ibn Al-Musayyab folding his hands in his Salah and he was always opening his hands during praying. Also in the same book (Al Musannif by Ibn Al-Shayeba Hadis No. 3974 ) a narrating from Yahya Ibn Saeed who is one of the very famous Ulama of Sunnis in Hadeeth, from Abdullah Ibn Al-Ayezaar who said: I was performing Tawaf with Saeed Ibn Jubayr who saw a man praying and putting his hand on the other hand, means folding his hands. Saeed Ibn Jubayr immediately went to that man and opened his hands, then came back to Tawaf. 
This is few examples from the Sunni Books that folding hands is not the Sunnah of Prophet. There is also Hadeeth in Al-Mussannaf by Ibn Shayeba “Kitaab As-Salat” Volume 1 Page 427. The Hadeeth is from the Prophet (s.a.w.a) informing the Muslims that (I can see the Monks of the Jews folding their hands during the Salah). This is clear that folding the hands is not the practise of the Prophet or His followers but the practice of others. Obviously, it was taken from some Muslim rulers from others and ordered Muslim masses to do it as you read in Bukhari ( People were ordered to fold their hands in Salaah)(Bukhari, 1:135).

2.Takbeer after Salah
The authentic evidence from Ahlulbayt (a.s.) and from others stated that the Prophet Muhammad Peace be upon Him and His Holy Progeny used to end every Salah by saying Allah Hu Akbar three times. It is in Bukhaari in the chapter of الذكر بعد الصلاة aHadeeth narrated from Ibn Abbas stating that the Prophet used to raise His voice by Takbeer at the end of the obligatory Salah. Ibn Abbas also said that we used to know the end of the Salah by hearing the Prophet saying Allah hu Akbar 3 times that is in Volume 1 Page 204 in Bukhaari and in Sahih Muslim Volume 2 Page 91 & 92. And in Sunan Abi Dawood Volume 1 Page 226 in Bab Al-Takbeer Ba’d asSalah. Shafi’ee in his Musnad narrated from Ibn Abbas that:  We used to know the end of the Salah by hearing the Prophet saying Takbeer 3 times. In Musnad Ahmad Ibn Hanbal narrating from Ibn Abbas theHadeeth that I did not know the end of the Salah but only by hearing the Prophet saying 3 times Allahu Akbar. Ibn Hajar Al-Asqalani in Fath Al-Bari, mentioned that this Hadeeth means that after Salah it is important to say 3 times Allah Hu Akbar. 
Al Tabari mentioned that the Prophet used to say 3 times Allahu Akbar after Salah. Ibn Maaja in his Sunan Volume 1 Page 280 mentioned that the Prophet used to raise his hands and after very Salah and say Allah Hu Akbar. Al-Tabaraani in Al-Mojam al-Awsat , volume 9 , Page 105 narrated from Qutada: I asked Anus Ibn Malik to show us how the Salah/Prayer/Namaz of the Prophet was? Anas stood and prayed the Salah and he finished the Salah with the Takbeer . He was teaching them the Salah of the Prophet. We also have the same evidence in Musnad Al-Humaidi Volume 2 Page 277 and the narration is from Abdullah Ibn Umar that he saw someone praying Salah and not saying Takbeer, Abdullah Ibn Umar got angry on him and throw him with a small stone telling him to raise his hands and say Takbeer Allahu Akbar. We have also in the book of Al-Tamheed by Ibn Abd Al Bar Volume 19 Page 134 that the Takbeer 3 times is a part of the Salah. I will end with al-Albani who is well known Wahabi Shaikh who mentioned in his book Itmam Al Minnah إتمام المنة Page 172 that raising the hands with Takbeer has been narrated  in authentic Hadeeths from the Prophet that he used to raise his hands and say Allahu Akbar.  
Wassalam.