Aluminum /6 Rear Main Seal

That rear main seal looks a bit smaller in width than the cast iron block version. It may be that seal is unique to the aluminum block. Silicone won't act as a good seal on a moving part. You may have to find an old school rope seal that you can cut to fit. A dab of RTV where it meets the end seals and pan gasket would be the only thing I'd use.
I'm not real sure what you mean by "uppper seal retainer". The upper half of the main seal should fit into a groove in the block. The lower seal half fits into a groove in the bolt-on seal retainer. The ends (right and left sides) of the retainer have a seal also where it meets the block. There again add a little RTV to the ends of the seals.