A couple of years ago, I was a member of a Gardenarian coven. It started out strong, but slowly dwindled away... Not to speak badly of the HPS, but her interest's lied elsewhere, not with her home, and the coven which she created.
I have a question though... when a coven ends, is there to be some kind of ceremony releasing everyone from the ties of the coven? Are the rituals still secret, never to be spoken or written about? Are we still oath bound? I feel that there was NO CLOSURE at all in this matter, and I'm wondering if there is such a thing as closure, when it comes to covens; even the inactive ones.
If it's done and over, then it's done and over. You can't be held to an oath you made to something that doesn't exist anymore. If it would make you feel better and more free, you could always ask the HPS to do some last 'goodbye' rite, or you could do one yourself with any other members who feel the same way. As for the rituals, I would still respect the secrecy of the specifics, especially anything that was unique to the coven, but honestly, most of the Gardnerian rituals aren't secret at all and have been written about a lot. Just don't gossip about things that happened during the rituals or about the other members, especially by name. Respect what's in the past, but you're not bound to it.
Speaking as a hedge-witch, and a non-Wiccan: the rituals are still secret, and your oaths still binding. However, if your coven has fragmented, you may consider starting your own coven, or joining another one. Whether you discuss what happened in your previous coven in your new one, is largely up to you. Perhaps best not to dwell on detail.
Why not do some grounding exercises and perhaps a ritual of your own for closure? Hedge-Witches would have no qualms about doing a solitary ritual for closure, if they felt it was right. What is to stop you?
The response to your questions will vary depending on who you ask.
I believe that all that was kept secret, should be kept secret, and a ceremony should be done. However this may be difficult, as covens lose members for all kinds of reasons. I do not think that a coven member will want to participate in a dissolution ritual, if they left because another member intentionally harmed them...etc.
Talk to the other coven members and see how they feel about it also. If you and one or some or even all coven members feel the same way then you should do a closure ritual, even talk to the HPS too. IF anything else start your own coven with the other members of the coven.