The word θέλω being pronounced as τσέλω is an acoustic error. There probably was some kind of complication with the program, since the voice is not a native speaking, but a Duolingo text-to-speech program. Just like trezost said, Θ is always pronounced like th is in "thanks", no doubt about that. ^.^
I don't think that's possible. The tiles from tile exercises are randomly picked from best translations, and ice cream is in the best translation. It might've showed up in one tile, but cream wouldn't be able to show up as an answer by itself. Are you sure you hadn't use it already, or just didn't notice it was there?