thermal battery and cob

As I build the TB from the ground up like " layering lasagna " I plan on building a structural straw cob retaining wall on the unsupported side of the bench ( other side is against a concrete wall )
1. Is this necessary for retaining a clay slip rock / brick / pea gravel TB ?
2. It's still not clear to me whether or not it is good to finish a TB with a layer of insulative material.
3. How thick?
4.Ratio of straw to clay slip by volume?

As long as the fill is going to be solid (no loose pea gravel), I don't think you need to worry much about a "retaining" structure. I wouldn't say an insulating facing would be helpful, but a structural cob mix with enough straw to make it strong would be a good semi-final step. (Followed by whatever finished surface you want to make.) Experiment with various ratios of the clay and straw you plan to use, and see how much you need for a good effect. You can always rewet the test materials and reuse them in the mass core, on the bottom if you think they are too insulative.