What about blending 2 HC? Don't you think they will blend to form an azeotropic gas in the end? Some of you use blend on cascade (methan+propan), so why would it be different here?

You're expecting to condense ethan or ethylen with respectively R600a or R290? Good luck

Considering they won't blend (I'm not sure they won't...) :
R170 is the hardest mid range second stage refrigerant (~-85°C @ 1bar like R23 and R13) to condense because of very high latent heat of vaporisation. A R600a compressor must have 65% greater displacement than needed with R12 to get the same capacity, so don't expect to remove the heat of R170 to condense it at decent pressures.

That's worse with ethylen and propan