Alternative to R508

[Shingoshi]

R-23 would be an only reasonable substitute for R-508B, period.
For single-stage auto-cascade you need at least two refrigerants, the so-called high temperature boiler and the low-temperature boiler. Two is - logically - the minimum

I should have gone to bed before asking that question. That's what I thought. I was just to tired to really think it through last night.

1.) So what other refrigerants could be used with R-23 in an autocascade?
2.) Why is solidification of R-744 an issue upstream of the capillary tube?
EDIT: The above question is a mistake. I meant to ask about upstream of the expansion valve.
3.) How are people using R-744 without having the icing problem?
4.) What other refrigerants would benefit from operating at the same pressures required by R-744?

Shingoshi