Yes, if I understood the programme notes correctly Stephens and his German colleague Sebastian Nübling drew on the experiences of Rinat Shaham as a opera singer who specialises in this one role. Shaham might also have combined the roles played in London by Sharon Small and Viktoria Vizin because I note that, in the play text, the cast for the Hamburg production lists only The Singer (Shaham) whereas in London The Singer is, rather confusingly, played by an actor (Small) with the actual opera singer (Vizin) listed as Chorus.
Indeed. I had a long exchange with Rinat last night on the play (and singing Carmen) and she performed as both actress and singer in the Hamburg premiere. She also pointed me this article: http://www.theguardian.com/music/2015/apr/13/carmen-as-male-prostitute-simon-stephens-carmen-disruption