[MunkDev]

I tinkered a bit more with this and you can actually do the same thing with just one button that uses a state driver, a wrapped PreClick and a custom identifier passed as the 'button' argument to the click script.

This solution only uses one secure button, does what you want, and the only drawback is that it won't trigger the first time if you're on a swap page when changing key bindings. This method also works when changing key bindings in combat.

local header = CreateFrame('Button', 'AllInOneActionButton', nil, 'SecureHandlerBaseTemplate, SecureHandlerStateTemplate, SecureActionButtonTemplate')
header:RegisterForClicks('AnyUp', 'AnyDown') -- process both press and release of an arbitrary key
header:SetAttribute('action', 1) -- this attribute will be used in conjunction with 'actionbar' to swap the bar back
 
do  -- generate a state driver condition with generic values to ensure any bar change pushes an update.
    -- the actual bar ID check is done in the _onstate-actionpage function body instead.
    -- this method seems to work regardless of API changes that cause
    -- some of these macro conditions to shift around on certain specs.
    local conditionBase, driver = '[%s] %d; ', ''
    local conditions = {
        ----------------------------------
        'vehicleui', 'possessbar', 'overridebar', 'shapeshift',
        'bar:2', 'bar:3', 'bar:4', 'bar:5', 'bar:6',
        'bonusbar:1', 'bonusbar:2', 'bonusbar:3', 'bonusbar:4'
        ----------------------------------
    }
    -- concatenate conditions into a long macro condition that can be
    -- evaluated by the state driver and send updates to the header.
    -- e.g. "[vehicleui] 1; [possessbar] 2; [overridebar] 3;" etc.
    for i, macroCondition in ipairs(conditions) do
        driver = driver .. conditionBase:format(macroCondition, i)
    end
    ----------------------------------
    -- append the list for the default bar (1) when none of the conditions apply.
    driver = driver .. (#conditions + 1)
 
    -- register the state driver to handle page updates
    RegisterStateDriver(header, 'actionpage', driver)
end
 
-- set up the _onstate-actionpage function body that sets the new actionpage ID.
-- this is a rewrite of the response in ActionBarController_UpdateAll.
-- state drivers use the _onstate-<name> attribute when looking for the function body to run.
-- 'newstate' is the argument passed to all state drivers expressing the outcome
-- of the registered condition, but in this case it's overwritten by the page calculation.
-- note that all code in a restricted environment is written as strings and compiled in
-- real time using loadstring(), to ensure you don't have access to unrestricted API.
header:SetAttribute('_onstate-actionpage', [[
    -- update actionpage
    if HasVehicleActionBar() then
        newstate = GetVehicleBarIndex()
    elseif HasOverrideActionBar() then
        newstate = GetOverrideBarIndex()
    elseif HasTempShapeshiftActionBar() then
        newstate = GetTempShapeshiftBarIndex()
    elseif GetBonusBarOffset() > 0 then
        newstate = GetBonusBarOffset() + 6
    else
        newstate = GetActionBarPage()
    end
    self:SetAttribute('actionpage', newstate)
 
    -- update bindings
    self:ClearBindings()
    for i=1, 12 do
        local key = GetBindingKey('ACTIONBUTTON' .. i)
        if key then
            self:SetBindingClick(false, key, self, tostring(i))
        end
    end
]])
 
-- wrap the preclick script on the header
-- use the inherent 'button' argument to tell the header
-- which action button you want to simulate.
-- since PreClick runs before OnClick, you're able to change
-- what the button should do while you're pressing it.
header:WrapScript(header, 'PreClick', [[
    if down then
        -- set up the downpress attributes to perform the action first
        self:SetID(tonumber(button)) -- see ActionButton_CalculateAction
        self:SetAttribute('type', 'action')
    else
        -- set the type to actionbar on release, which swaps the bar back
        self:SetAttribute('type', 'actionbar')
    end
    self:CallMethod('OnButtonPressed', down, button)
]])
 
-- finally, execute the actionpage response on load,
-- to set the correct action page and your override bindings.
-- 'Execute' is included in the SecureHandlerBaseTemplate,
-- and allows you to execute code in a restricted environment.
header:Execute(header:GetAttribute('_onstate-actionpage'))
 
-- this function is called from the preclick script,
-- which visually indicates a button press on your bar.
-- this part isn't functionally necessary,
-- it just makes it look like you're pressing regular buttons,
-- when you're actually pressing your hidden button.
function header:OnButtonPressed(down, id)
    local button = GetActionButtonForID(id)
    if button then
        if down and button:GetButtonState() == 'NORMAL' then
            button:SetButtonState('PUSHED')
        elseif not down and button:GetButtonState() == 'PUSHED' then
            button:SetButtonState('NORMAL')
        end
    end
end