I think, how it works would be that upon asking to be released from the oaths, the domains would return to the holder's superior.
It gets complicated.
From what I understand, it seems to be like:
A region of space, is under the jurisdiction of one of the Heir families. The Heir can appoint people to constellation governors and system governors, and possibly even micromanagement down to subdivisions of countries on planets. Although I think there'd be a degree of delegation, so a constellation governor can appoint people as system (or parts of system) governors.
e.g. in the Amarr cosmos area, we have:
Uriam Kador - Region lord.
Manel Kador - governor of Araz constellation
Lords Arachnan and Methros - own things in space in Araz
Baron Maphante of Garisas - a minor Holder in Garisas
And then there's subdivisions, e.g. Lord Arachnan has a sister, who has her own estates.
The higher levels of holders can, I think, retain elements of a domain purely for themselves.
E.g. Uriam Kador grants Manel governorship of Araz. There may be estates on the planets of Araz that are not under Manel's authority, having been retained by Uriam for his own uses.
Holder families can trade lands, gain them or lose them, through one of the popular pastimes of Holders: Intrigue!
Which is possibly how you end up with some of the oddities like the landless minor Holders. Though there are also things such as grants of titles and estates to Navy veterans, for example.
And that's just the land ownership. Land owners can I think, then rent elements of their domains out to commoners for economic purposes, and those agreements would be yet another convoluted scheme.