How about the +on vs +true. DomoPad seems to require the +true while Web + iOS app require a +on
That's not quite accurate - I think what you are seeing there is due to your first test... things behave differently whenever you use the + versus the non-plus version (which Domotics Pad does not support).
Basically, Indigo Touch and the web have some form of implied heuristics that can do simple image selections such as on/off and true/false. This is invoked whenever you have an image WITHOUT a + sign to end the name. Domotics Pad does not support this method/version (it was the "old" way prior to about Indigo v4, Domotics Pad wasn't around then).
Domotics Pad supports the more precise image heuristics as described
in the Wiki. In this method, which is triggered with the use of the image with a trailing + sign, the server will attempt a direct match of the value after the plus sign, then the server will make other attempts if no exact match occurs. This method comes from the server and
should behave the same on all clients.