Normal etiquette dictates that mess kit is evening wear and should not be worn before 1800. Despite most mess kits having a black tie, it is actually the equivalent of the civilian "white tie" order of formal dress, which also includes tails. A tuxedo (with its black tie), by the way, is considered semi-formal. We no longer use mess "undress," which would be the military equivalent to a tuxedo.
Strictly speaking, a daytime wedding ceremony should be conducted in No 1 (ie. uniform shirt, standard tie, medals and sword) or 1A (same as above without sword). An alternative would be if you are a member of a regiment that has a regimental uniform (eg. a red-coated infantry regiment).
There is no strict taboo against wearing swords in church. People have been doing it for centuries. You may wish to check with the church in question though as they may have a local policy. I wore a sword (as did my ushers) for my wedding and it looked great! You just have to be careful when moving around, especially if there is any kneeling involved. My wedding was in a Catholic basilica, with a full mass and lots of stone steps. The arch of swords (edges up) is a nice touch and makes for great pictures.
It is OK to wear mess kit for the reception, if it takes place after 1800, but their is no requirement to do so. I have been to weddings where people changed into mess kits and others where they didn't. As a personal recommendation though, if you don't have or don't want to buy a proper mess kit, I wouldn't bother changing into a white shirt and bow tie and swapping medals for ribbons (medals are not worn with interim mess kit - miniatures are worn with proper mess kit). If you want to wear a mess kit at your wedding, spend the bucks and get a proper one. It looks better and you'll be glad you did when you see the pictures later. If you're worried about the cost, just consider what your bride will spend on her dress. Shouldn't you be allowed to do the same in this modern world of equality of the sexes? Did I just say that!?
For my wedding, my bride bought a dress and I bought a sword (I already had the uniforms).
Whatever you do, DO NOT wear a sword with mess kit (yes, I've seen it done and it is WRONG, WRONG, WRONG). If you're going to cut the cake with your sword (which is really cool) and you're wearing mess kit at the time, just have the sword placed on the table beforehand. Don't wear it. On another note, make sure you clean off the sword both before and after cutting the cake. The icing will do a number on the blade finish.
Congratulations and good luck.