FYI, I have worked on this in the past. Note that SNMP querying is not fully supported. By that I mean, even though the PUBLISHED MIB has OIDs, there are a significant number of them that actually do not work.
With that, you are better off using the "event" system in ONTAP and letting ONTAP send alerts out to anything (SNMP, Mailhost, syslog)
There is an Express Guide to set that up on the ONTAP 9 docs site.
I know that does not answer the questions you are asking but I thought it might be useful before you go too far.