From the privileged position of having seen both forms of the invite, I don't know how you could've made it any clearer. If the invite doesn't give a ceremony venue, the recepient ought to figure out that they are expected not to turn up. Maybe the solution is to beef up the "Numbers […] are restricted" on the website to make it clear that if someone without a ceremony invite turns up they won't get in.

Anyway, you have my complete sympathies regarding trying to work out how to present all the necessary invite information without requiring large letter postage.

And we really should R.