do you mean the functions here?
https://api.ovh.com/console/#/domains
I'm not sure what you mean by that. These functions allow you to run the same actions that you have from the manager. The DNS servers for a particular domain depend on the DNS servers where the DNS zone has been created. Those will have to be the one to use.
If you meant something else, please clirify.
For question 2) , I guess you'll have to use Glue records. At the moment, you'll have to do that through the managerv3