I think that original 's' was a good start, only it looked too much contructed. The upstroke could be more slanted. Imagine how it looks in upright fonts — it usually is not "upright" as well, but already slanted — so I think it looks more genuine if the italic version is slanted more than the italic stems. It could also be slightly curved.
By the way, I think the upstroke connecting the s strikes as too thin (compared to the thinnest in 'd', for instance).
How about including duplicates of the affected glyphs (with one unencoded and one PUA-encoded slot per glyph)? Would that make everyone happy? Of course the file size would soar. Or maybe two separate font files could be created with clear explanation for the customers. Certainly, this last solution doesn't seem too crisp.
I'm not sure having the bottom curve of the /a tangent to the vertical is what you want. You might want to move the control point to the left to let more light in. Conversely, you can try working with separate shapes for each element of the letter. This way you will have greater control over the pen logic of your design. The idea: