1.Exchange 2010 SP2 RU6
2.Yes anything 20MB and lower goes without issues it seems to be right about 20400KB where messages will not go (my presumption is MIME + headers may make it too large)
3. This has been on going since deployment.
4. Microsoft Hybrid Coexistence deployment assistant: technet.microsoft.com/.../default(EXCHG.150).aspx
Plain text / HTML emails route fine it is only when an attachment or attachments are included where the message size hits 20400kb.