Wow, that is a big pile of rude! If it's true--it sounds like there is enough evidence to suggest that it is, but I would want one of them to confirm it to me directly (not that I would ask). If it's true, I would definitely feel used for money and lied to.
There could be lots of reasons they opted for a commitment ceremony over a wedding and I personally wouldn't necessarily have treated it differently--kind of depends on the reason, if they chose to share it. But I think it should be clear to people what they're witnessing--it sounds like they went out of their way to make people believe they were witnessing a full, legal wedding (or at the very least that one had/would be happening around the same time). Worst case scenario, they knew they couldn't be in that government program if they were legally married (I'm guessing it leads to some kind of financial benefit), but also knew their guests wouldn't be as generous if they had a commitment ceremony only for that reason, so they deliberately lied to get money from both their guests and the government.
It's awkward enough to know that one is planning an anniversary party, while the other is thinking of hightailing it. If there was only this issue--I guess I'm not sure. On the one hand it could be that the party is a trainwreck and I'd be afraid I was attending out of prurient interest, so perhaps I should decline. On the other hand, if one person is clueless that their relationship is on the brink of dissolving, maybe they need me to be supportive of them now more than ever.
For me personally, before I treated them differently over the non-marriage thing, I would need to know for certain, as in getting confirmation from one of them directly. In a general sense I don't think it would be polite to ask; but there are some people I feel close enough to (mom, sister, daughter level) that I would ask anyway. I don't know if you feel that close to them or not. I would hate to distance myself from someone I previously cared for over issue X, only to find out that issue X was all a big misunderstanding.
ETA: The OP says it's definitely true. So in that case I would let them drop way, way off my radar. Maybe not a full-on Cut Direct where I'd ignore them if they said hi on the street, but I would be turning down all invitations and keeping any conversation with them brisk and cool.