ReAction, what is missing to make it a good GUI?

Tags should describe object properties, not invoke function calls - that's what methods are for. So the introduction of the child removing method was the right step, in line with OOP principles.

if(LayoutBase->lib_Version>=53)
{
IIntuition->IDoMethod((Object *)mommy, LM_REMOVECHILD, inst->Win, ubrow->buttons[j], TAG_DONE);
}
else // Use fallback function. Note, this function may deadlock
{
IIntuition->RefreshSetGadgetAttrs(mommy, NULL, NULL, LAYOUT_RemoveChild, ubrow->buttons[j], TAG_DONE);
}

DoMethod(group, MUIM_Group_InitChange);
DoMethod(group, OM_REMMEMBER, button);
DoMethod(group, MUIM_Group_ExitChange);

if you want real oop you should have:

mommy.RemoveChild(j)

this should be the right step...