Impossible to use custom implementation of public BCS interface IEntityInstanceProvider

SharePoint Business Connectivity Services contains out-of-the-box webparts to visualize external business data. All the standard BCS business data renderers utilize the pattern that data selection is delegated to a data provider control. The 2 controls are associated via consumer/provider connectionpoints. BusinessDataDetailsWebPart is one of them. Its standard usage is for BCS profile pages. However, usage of this webpart on itself is not necessarily limited to that context alone. The webpart is selectable in the webpart gallery, and can be added to arbitrair SharePoint webpart page. The challenge is how to connect it to an external BCS entity. The trivial approach is to do it similar as on profile pages; feed it via the provider BusinessDataItemBuilder. The working of BusinessDataItemBuilder depends on an ‘ID’ querystring parameter. However, when that parameter is included in the url of a page with the 2 BCS webparts on it, SharePoint faults with an error message:

System.NullReferenceException: Object reference not set to an instance of an object.
  at Microsoft.SharePoint.Publishing.Navigation.PortalSiteMapDataSource.OnInit(EventArgs e)

This occurs because PortalSiteMapDataSource also reacts on the presence of ‘ID’ querystring parameter, but assigns another and in this context incorrect meaning to the value. When the ‘ID’ querystring parameter is renamed, the error is prevented. However, then the standard BusinessDataItemBuilder no longer reacts on it.

A proper solution seemed to build a custom provider that relies on another named querystring parameter to retrieve the BCS entity item. The BCS architecture has the promise to support this: it heavily decouples via interfaces from the specific BCS implementation classes. But this appears to be a false promise. Microsoft already points you to that via the warning accompanying the public (!!) interface IEntityInstanceProvider

This class and its members are reserved for internal use and are not intended to be used in your code.

Still, since the interface is public; and also the connectionpoints are public; it sounded safe to expect it must work. But when I added both my own IEntityInstanceProvider implemenation class and a BusinessDataDetailsWebPart on a page; it was not possible to connect them; not in the GUI, and also not explicit in custom code. Via Reflector I reverse engineered the cause, to ultimately end up with the following:

Microsoft.SharePoint.WebPartPages.WebPartPageUserException was unhandled
Message=The connectionpoint BDWP Item on g_1e1e7d15_652d_44cd_bc7d_9026382ff33b is disabled.
Source=Microsoft.SharePoint
StackTrace:
at Microsoft.SharePoint.WebPartPages.SPWebPartManager.CanSPConnectWebPartsCore(WebPart provider, ProviderConnectionPoint providerConnectionPoint, WebPart consumer, ConsumerConnectionPoint consumerConnectionPoint, WebPartTransformer transformer, Boolean throwOnError)

and digging deeper:

if (!providerConnectionPoint.GetEnabled(connectionAgentByInterfaceName))
{
    if (throwOnError)
    {
        throw new WebPartPageUserException(WebPartPageResource.GetString("SPWebPartConnection_DisabledConnectionPoint", new object[] { providerConnectionPoint.ID, provider.ID }));
    }
    return false;
}

public override bool GetEnabled(Control control)
{
    bool flag = false;
    BusinessDataWebPart part = control as BusinessDataWebPart;
    if (part != null)
    {
        try
        {
            return part.CanConnect(true);
        }
        catch
        {
            return false;
        }
    }
    if (control is BusinessDataItemBuilder)
    {
        flag = true;
    }
    return flag;
}

Thus: although at the external front decoupled via interface IEntityInstanceProvider, internally BusinessDataDetailsWebPart demands it's provider to be either a BusinessDataWebPart or BusinessDataItemBuilder webpart!! So much for interface decoupling.
Confine to this internal constraint by inheriting either of these classes is also not possible: BusinessDataItemBuilder is sealed, and BusinessDataWebPart is public abstract, but with 4 internal abstract methods.

BusinessDataDetailsWebPart also allows to explicit connect it in code with a BCS entity. Less elegant as via the connectionpoint, but workable. The standard BusinessDataDetailsWebPart can then still be used to render the external data item; no need to custom build an own one (or ‘copy’).

BusinessDataDetailsWebPart busDetailsWp = wp as BusinessDataDetailsWebPart;
if (busDetailsWp.BdcEntity != null && busDetailsWp.BdcEntity.Equals(entity))
{
    string entityInstanceId = ((IEntityInstanceProvider)this).GetEntityInstanceId();
    busDetailsWp.ItemID = entityInstanceId;
}