I believe the issue is not about following the imam of another madhhab. The issue is when the imam does something that invalidate salah of the ma'mum of another madhhab but not the imam's, as given in the example above (i.e. bleeding while praying or when you see a Hanbali imam wipe wudhu' on socks).
The problem here arises when we say that they are all valid ways, and we then go on to say that prayed behind such and such a person is void. Do you seriously think that Imam Shafi'i while holding the opinion that Basmala is Fard in Fatiha wouldn't pray behind Imam Malik who beleived it is Makruh. As long as it's opinions based on sound understanding and knowledge, and not whims and desires. Yes we may find such fatwas in some books, but that doesn't mean they are necessarily correct. Just as major Imams have said that one can't marry outside the school etc. Such extremism wasn't uncommon at certain times in Islamic history. Let us not repeat their mistakes.
Shaykh Abdul-Hayy Lucknowi said that when a hanafi pray behind an imam of another madhab, his prayer is legally valid if his prayer was valid according to his school of thought, even if the imam did something that could annul the muqtadi's hanafi prayer. The shaykh then quotes Abu Bakr ar-Raazi (another hanafi 'ulama), who said that if an imam from another school of thought bleeded and didn't make his ablutions again (as per his madhab's rulings), and then led prayer for a hanafi then the latter's prayer is legally valid (Al-Hidaya, 1/126 end of page). That is why Abu Yusuf prayed behind Hârûn ar-Rashîd when he just had done a bleeding done on him as per Maalik's fatwa given to him. (Sharh ul-’aqîda at-tahâwiyya, Ibn Abi-l-’izz al-hanafî, 2/535). Shah Waliyyullah wrote: “Among the sahabah, the tabi'un and their followers, some used to recite the basmala (during the prayer), others would not; Some would recite basmala loudly (during prayer), some others not ; others making qunût in salât ul-fajr, others would not; some would consider ablutions cancelled by a bleeding, nose-bleeding or vomiting and others would not consider ablutions cancelled by one of these reasons; (similarly with touching the genital parts or his wife with pleasure, , or by eating what was touched by fire or with eating camel food). In spite of this, they would pray behind one another. Thus, Abu Hanifa or his 2 pupils, and ash Shafi'i and others would pray behind imaams of Madina etc., although they would not recite basmala loudly nor silently. Ar-Rashîd led prayer after having done a bleeding and Abu Yusuf prayed behind him and din't repeat his prayer. Ahmad ibn Hanbal thought ablutions were cancelled by nose-bleding or bleeding and when questioned he said" Would I not have prayed behind Mâlik or Sa’ïd ibn ul-Mussayyib ?”" (Hujjat ullah il baligha,Volume 1 pp. 455-456). Warning: Errors of translation likely!
Asslamu alaikum brother, I understand wiping 3 hairs is enough in Shafi fiqh. But this is the minimum. The sunnah is to wipe the whole head and there is general agreement regarding this. So tell me brother why would someone intentionally leave out the sunnah? If you read Bustan al-Arifeen you will see how strongly Imam Nawawi himself talks about following the sunnah. So unless we see someone wiping only 3 hairs we will assume they followed the sunnah and did the whole wudu and whole ghusl. Sunnah of ghusl is to do wudu and sunnah of wudu is to put water in the nose and gargle and rinse the mouth.