"Reading the fine print on some registration/hosting sites, it appears that with many, you only hold title (as it were) to the domain name as long as you're a client of theirs"

There are some like that. Avoid like the plague. There are plenty of others that aren't. Also you can keep one company as your registrar but move your hosting somewhere else, though they'll often try to persuade you to keep registration and hosting under the same roof.

Now, about IP addresses:
The Domain name database lists a primary DNS (server) for you domain.
That DNS says what IP addresses (there may be several) are associated with that domain.
As the domain owner you can change which primary DNS you use.
The hosting company usually controls the contents of the DNS server.

I'm not sure what all this about "repositories" is supposed to mean - are you talking about the pools of dynamically allocated IP addresses that are used for many dialup and broadband connections? They have very little to do with the domain registration system.

Anahata