From you description, I'd argue that we can't do much here, that's EC's job.
EC should disable the touchpad wakeup when it detects lid is closed, but sounds like your laptop only disable touchpad wakeup when both lid is closed and touchpad is touched.
Instead of
if (lid_is_closed)
disable_touchpad
Its logic sound like
if (lid_is_closed and touchpad_in_use)
disable_touchpad
IMO this bug is not related to linux and should report to Dell.
From you description, I'd argue that we can't do much here, that's EC's job.
EC should disable the touchpad wakeup when it detects lid is closed, but sounds like your laptop only disable touchpad wakeup when both lid is closed and touchpad is touched.
Instead of
if (lid_is_closed)
disable_touchpad
Its logic sound like
if (lid_is_closed and touchpad_in_use)
disable_touchpad
IMO this bug is not related to linux and should report to Dell.