Is stipulating marriage according to the Madhhab of Abu Hanifa al-Nu'man during the mawzun's تلقين (instruction/dictation of the contract) considered invalidating the contract or makrooh (reprehensible), and what should be done about this custom?

There is no obligation to mention the madhhab (school of thought) in the marriage contract, nor should it be explicitly stated. However, explicitly stating it does not invalidate the marriage. So, if a marriage contract were concluded in imitation of Abu Hanifa, allowing a mature woman to marry herself, the contract would be valid due to the existence of scholarly disagreement. The majority of jurists hold that the guardian (wali) is a condition for the validity of marriage, and this opinion is the strongest due to the strength of its evidence. Mentioning a specific madhhab in the contract does not necessitate following it in all matters of marriage and divorce, for a Muslim is permitted to follow other scholars, as "the common person has no madhhab; rather, his madhhab is that of his mufti."