Have you tried what will happen with a enabled backaddress and no value for backaddress, fromname and fromaddress are shown as backaddress but I don't rely on this feature and change it to waht you suggested.
I've just tried this quickly and this seems even better … then what about removing the option "backaddress=yes" from "\setupletterstyle" and provide a key "auto" for "backaddress" in "\setupletter"? Thus:
1. No backaddress line
\setupletter [fromname={Arthur Thor}, fromaddress={Hidden Street 2\\57895 Mysterytown}]
2. backaddress line generated from sender information
\setupletter [fromname={Arthur Thor}, fromaddress={Hidden Street 2\\57895 Mysterytown}, backaddress=auto]
3. Custom backaddress
\setupletter [fromname={Arthur Thor}, fromaddress={Hidden Street 2\\57895 Mysterytown}, backaddress={Thor Inc., Mystery Street 5, 59875 Hidden Town}]
In this case there would be no need for "name=yes" etc. Oliver