I can answer my second and third point; there's an IMetadataExtender interface that can do whatever it wants with the metadata. Some reflection is needed to access the attributes, but I see no issues with this.

I still consider this as a bug, but now I have a workaround that I'm satisfied with.

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web.Mvc;
using EPiServer.Core;
using EPiServer.Framework;
using EPiServer.Framework.Initialization;
using EPiServer.Shell.ObjectEditing;

[InitializableModule]
internal class MetadataExtenderModule : IInitializableModule {
    public void Initialize(InitializationEngine context) {
        var registry = context.Locate.Advanced.GetInstance();
        registry.RegisterMetadataHandler(typeof (ContentData), new ContentDataMetadataExtender());
    }

    public void Uninitialize(InitializationEngine context) {
    }

    public void Preload(String[] parameters) {
    }
}

/// 

///     

///         An IMetadataExtender which reapplies values that EPiServer has overwritten in
///         ContentDataMetadataProvider.GetContentDataMetaData. This affects all values
///         where EPiServer expect the LocalizationService to handle translations.
///     
///     

///         In case of DisplayName it will fallback to the property name, which comes
///         from the neutral resource file. We reset it to a call to DisplayAttribute.GetName
///         so we get the text for CurrentUICulture.
///     
/// 
public class ContentDataMetadataExtender : IMetadataExtender {
    public void ModifyMetadata(ExtendedMetadata metadata, IEnumerable attributes) {
        foreach (var propertyMetadata in metadata.Properties) {
            var property = propertyMetadata.ContainerType.GetProperty(propertyMetadata.PropertyName);
            if (property != null) {
                var propertyAttributes = property.GetCustomAttributes(true)
                                                 .OfType()
                                                 .ToArray();
                ModifyMetadataFromAttributes(propertyMetadata, propertyAttributes);
            }
        }
    }

    private void ModifyMetadataFromAttributes(ModelMetadata metadata, Attribute[] attributes) {
        ModifyFromDisplayAttribute(metadata, attributes.OfType().SingleOrDefault());
    }

    public void ModifyFromDisplayAttribute(ModelMetadata metadata, DisplayAttribute attribute) {
        if (attribute != null && attribute.ResourceType != null) {
            metadata.DisplayName = attribute.GetName();
            metadata.Description = attribute.GetDescription();
        }
    }
}

// Simon