CPanel DNS Clustering And Redundancy

I am working on setting up a few servers that run cPanel and the usual software, minus any sort of DNS server. These servers are setup to sync all DNS records over to a cPanel "Dns Only" (ns1.domain.com), which in turn syncs all its records to a backup DNS server (ns2.domain.com).

In other words:
[Domain registrar]
| |
[NS1]--Sync-->[NS2]
|
(Sync to NS1)
|
Cpanel Web Server

Now, say something happens to NS1 and the sever goes completely offline (i.e. power supply dies, CPU goes bad, etc.), which of the following scenarios would actually happen:

1) Because the registrar lists both NS1 and NS2 as NS records, NS1 would time out and the DNS lookup would look to the secondary DNS (NS2) for the record.

2) The registrar would randomly give out NS1 OR NS2 because of round robin, and if NS1 is given to a client as the result of a DNS lookup, the site will appear down, however if the client happens to be referred to NS2, the site would appear online.

3) The site would be down no matter what.

So, if someone with knowledge on the subject wouldn't mind enlightening me as to which of those would actually take place in the event of a failure on NS1, and maybe some suggestions as to keeping the DNS truely "redundant", then I would greatly appreciate it.

And Im aware that there are many 3rd party services that will take care of the DNS records and make them redundant (DYNDNS, DNSMadeEasy, etc) But I would prefer to keep the DNS in our full control.