Qiraat in Salaah

Q: After reciting Surah Ikhlaas in salaah, I mistakenly said sadaqallahul azeem. Is my salaah valid or is there sajdah sahw?

Q: When you want to read a big surah in salaah, but not in full. Is it okay to read from anywhere in the surah and stop anywhere? And can you read one part of a surah in the first rakaat, the part after that in the second rakaat? Or do you have to read different surahs in each rakaat? 

Q: As we know it is waajib to recite the first 3 verses of Surah Faatiha in every rakaah of salaah.

I heard a mufti say that qiraat changes the amount of verses in the Quraan. If I recite the first three verses of Surah Faatiha and combine the second and third verse and not recite further would my prayer be valid?

Q: What's the sunnah method of reading Surah Faatiha in salaah? Each ayah a different breath? Or joining more than one ayah in one breath?

Q: How many verses of Surah Fatiha is waajib to recite in salaah?

Q: During my witr salaah, sunnah muakkadah salaah and nafl salaah, how loud can I read qiraat? Can it be as loud to the extent that people close to me can hear me?

Q: Is there any qawl amongst the ulama which stats that the sunnat qiraat in salaah is not which surahs are recited but rather the amount recited?

Q: Besides reading a complete Surah or atleast three ayaats of the Quaran after Suratul-Fatiha in salaat, I understand one long ayaat can instead too be read. But how long should the ayaat be and on what basis is the length calculated?

Which long ayaats (if any) are generally deemed preferable to read?

Q: With regards to Friday Fajr salaah: If I read Surah Sajdah in 2 rakaats one week, and the next week read Surah Dahr in 2 rakaats, will I get the fadheelat of the sunnah? Because the musallees are old, and they do not like to read long salaah. For this reason, can I get the reward for the sunnah?

If there's no sunnah reward for this, then how can I get the sunnah reward?

Q: Can I repeat the same Surah in each rakaat of salaah?