OP Very sorry for your loss.
If you can get married legally a few days before, that would be the simplest. It's perfectly usual in other countries to speak of a 'civil wedding' and then a 'religious wedding'. As previous posters have said, no-one is remotely fazed by that. So you could invite people to your 'humanist wedding' or your 'wedding ceremony' and be telling the absolute truth. No deception. (Any anyone who complains will be very mean, especially at a time when you are still grieveing.)
But if that still troubles you, so long as the register office is not far away, I think the idea of leaving the party for half an an hour is a good one. Or, even better, if you can arrange for the Registrar to attend your chosen venue, the legal formalities only take a few minutes.
That way, the whole day becomes your 'wedding day', and from what you say, that is clearly very important to you. You'll be exchanging personal vows - you say they'll be the most meaningful ones- and then legally registering them. All on the same day.
Obviously, what matters is how you and your soon-to-be-husband feel, but, for my own part, I found saying the legal vows (at at very quiet wedding on a grey Wednesday morning in a drab concrete register office!) very moving and not a little awe-inspiring. You're making a binding promise, in front of witnesss, to link your destiny and welfare to another's, for good or ill.