What are the condition needed for a qira'at to be valid?
Imam Ibn al-Jazari said in the beginning of his book النشر في القراءات العشر:
كل قراءة وافقت العربية ولو بوجه ووافقت أحد المصاحف العثمانية ولو احتمالاً وصح سندها فهي القراءة الصحيحة التي لا يجوز ردها ولا يحل إنكارها بل هي من الأحرف السبعة التي نزل بها القرآن ووجب على الناسب قبولها.
Every qira'at that fits with Arabic Language (even if from single grammatical point of view) and matched an Uthmani Mushaf and had a reliable narrators chain, is a valid qira'at and it's not permitted to reject or refuse it, and it's one of the seven tongues the quran was revealed in, and people must accept it.
He followed that with an explanation, which I'll try to summarize and translated:
Fitting Arabic Language: It should fit the grammar of the Arabic Language, and since there are many schools in the Arabic Language grammar, it should fit at least one grammatical point of view, a single point of view is enough if the qira'at fulfills the third condition.
Matching one of the Uthmani Mushafs: as there are some minor differences among the Uthmani Mushafs, matching any these Mushafs fulfills this condition, if the qira'at didn't match any of them, it is no longer considered valid. Please not that this condition has more details I can't really explain in English, so I didn't mention.
Having reliable narrrators chain: So it should be narrated by a chain of reliable and dependable people (رواة عدول), and be well known for the reliable reciters قراء. If it wasn't, then it's not valid. This condition is the strongest among all three conditions, it's the base condition.
Who coined these conditions?
These conditions are agreed upon by all Quran and verification scholars. Imam Ibn al-Jazari mentions in the beginning his book النشر في القراءات العشر (right after the above quote) some of the notable scholars who agreed on these conditions:
هذا هو الصحيح عند أئمة التحقيق من السلف والخلف، صرح بذلك الإمام الحافظ أبوعمرو عثمان بن سعيد الداني، ونص عليه في غير موضع الإمام أبو محمد مكي بن أبي طالب وكذلك الإمام أبو العباس أحمد ابن عمار المهدوي وحققه الإمام الحافظ أبو القاسم عبد الرحمن بن إسماعيل المعروف بأبي شامة وهو مذهب السلف الذي لا يعرف عن أحد منهم خلافه.
And on that agrees the previous and coming verification scholars, that's what the preservative Imam AbuAmr Othman bin Sa'id Addani stated, and said it Imam Abu Mohammad Makki bin Abi Taleb, and also Imam Abu Abbas Ahmad Ibn Ammar Mahdawi, and verified it the preservative Imam Abu Qasim AbdulRahman bin Ismail (Abu Shamah) and that's the approach of all predecessors, none of them was told to take a different approach.
Hope that's clear (especially translation), and Allah knows best.
Also this is a good online source, unfortunately it's in Arabic: , .