1) There is a Hadith saying that whoever prays 12 Raka'hs a day, Allah builds a palace for him/her in Heaven. Does the Hadith refer to 12 Raka'h everyday or only one day? 2) There is a Hadith narrated in Imam Ahmed saying that reciting Surah al-Ikhlas 10 or 11 times lead to a palace being built in Heaven. Is this Hadith Sahih? 3) There is a 4 Raka'h Sunnah before Zuhr prayer; if I missed them can I make them up after the prayer?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The Hadith you mentioned means that whoever performs twelve Rak'ahs of the Sunnah prayer during a day and night, Allah will build for him a house in Paradise.
So, a Muslim is highly advised to perform such Rak'ahs everyday so that he gets a new house in Paradise. This, in facts, is one of Allah's Favors on His slaves. This is why Ummu Habibah (Radiya Allahu Anha) said: "I did not leave them since I heard about them from Allah's Messenger (Sallallahu Alaihi wa Sallam)" . [Reported by Imam Muslim ].
As for the Hadith that reads: "Whoever recites Surah of Ikhlas eleven times, Allah will build for him a house in Paradise" , it is sound. It is reported by Imam Ahmad and al-Sheikh al-Albani regarded it as sound.
If one misses the stressed prior Sunnah of al-Zuhr or any similar prayer, it is legal to perform it.
Umm Salamah (Radiya Allahu Anha) reported that the Prophet (Sallallahu Alaihi wa Sallam) "Performed two Rak'ahs after al-'Asr, and when she asked him about that he replied: "Some people of Banu 'Abdul Qais came to me professing Islam and made me busy and forget about the subsequent two Rak'ahs of al-Zuhr, and these are the two Rak'ah I performed after al-'Asr" . [Reported by Imams al-Bukhari and Muslim ].
Another evidence that proves the legality of performing the missed stressed Sunnah prayers is the Hadith reported by Abu Sa'id al-Khudri that the Prophet (Sallallahu Alaihi wa Sallam) said: "Whoever sleeps through the time of his Witr prayer or forgets it, then he should perform it when he recalls it" . Abu Dawood , and al-Nawawi graded it as good.
Allah knows best.