You don't need any MIB. Regarding OC 5.1 trap listener, is there any other application holding port 162? can you check OnCommand Events Page for warnings. If port 162 is bound to some other application DFM can't receive any SNMP traps.
AFAIK the OnCommand Core server historically only acts on SNMP traps originating from ONTAP storage controllers (and formerly NetCache appliances). It will disregard SNMP traps coming from any other device.
Good question. The immediate answer is "No". That said, I have passed your question onto some other folks that might now a bit more about the integration between WFA and OC UM 6.x, where I believe that capability will land. More information to come when we learn more from the product / engineering teams.