Is it the correct behavior that "show snmp mib | include ifInErrors" doesn't return anything? I do see a bunch of "ifEntry.#" and
with "show snmp mib | include if"
Yeah. The way IOS builds the internal MIB tree doesn't mean that each object will be translated to it's actual object name. Many times, the object will be referred to be its table entry, then index in that table, or by the MIB, then the table, then the object.
You can use my SNMP translate tool at http://jaguar.ir.miami.edu/~marcus/snmptrans.html to do bulk file translations. This should handle most of the objects show snmp mib returns.