Re: put a NS of a domain
You seem to misunderstand the concept of redundancy with regards to DNS. Adding additional failure points is not "redundancy".
If you have all the servers for example.com under example.net then this should be a well considered decision because a failure of EITHER example.com or example.net will cause problems. However if you are talking about dedicated DNS servers then putting them all under example.com is the most optimal configuration in terms of both performance and reliability. Queries will go directly instead of resolvers having to do an entire extra DNS traversal, and the only thing you rely on is the parent servers for .com - if they go down then example.com isn't resolving, is it?
A TLD should not be dependent on anything except the root zone, using another TLD will either involve delays in resolving your TLD or will require that you get authority to add glue records to the root under someone elses zone. If you do the latter it would be functionally equivalent to using your own TLD, with added paperwork and failure points.
In short, "Using the same TLD" is only a silly idea if that TLD is a different one to the one the domain is under. Putting your name servers under your own domain, on the other hand, is the correct and best way to do it.
Of course the question then becomes what the root referrals looked like - correct IPs, or his IPs?