How can one ascertain whether an imam who substitutes one letter for another in prayer does so intentionally or not, and is it necessary to investigate this, especially concerning the substitution of "dhāl" (ذ) with "dāl" (د)? And does this fall under the excuses accepted by the later Hanafis?

The Maliki school of thought holds that it is valid to follow in prayer someone who unintentionally makes errors in reciting Al-Fatiha, even if he is ignorant and open to correction. Some later Hanafi scholars opined that errors in letters do not invalidate the prayer if there is a widespread affliction among people, or if the two letters do not share the same articulation point or a close one, such as replacing the "dhāl" with "dāl", and that the default assumption is that the error is not deliberate.