Graham Thompson

I think you'll probably need to provide an example file of what you're trying to do since there should be no issue running callbacks via non-graphical Houdini and a quick test results in the expected behavior.

In terms of catching errors related to pressButton(), you'll never be able to. While calling it is done in Python, there's no guarantee that the callback is executing Python so there's no real expectation that any errors experienced during the callback would bubble up as something useful. It's mildly inconvenient that it doesn't throw some general exception like hou.OperationFailed when something seems to go wrong though.

Since you're creating the attribute in VEX you need to manually create a mapping between the parameter and a variable:

That being said, PolyExtrude won't accept a local variable in that parameter so even if your variable was correctly set up it would throw a warning. In your case you could do something like set the Distance to 1 and use your attribute as the Distance Scale in the Local Attributes tab.

Yeah, those ones look to be defined in specific .shelf files in $HH/toolbar so there's not much that can be done about them other than making copies of the tool definitions and trying to remove all their context settings to not show up.