Actually what you said at the start and finish is that you believe SE could already do it if they wanted to...
Reason why it doesn't work is because SE doesn't want it too. Simplest way i can say it.
Can the effects of multiple additional effects stack? My gut says prolly...if SE ever allowed it.
where as I think they cant, as they basically said in that post up above...
changing the priority so that Excalibur's added effect is higher than Enlight would require a complete reworking of the weapon as well as limit the use of Enlight
To me this implies they can not simply allow both effects, one has to have a higher priority, in this case, Enlight.