First things first; Stop worrying about it. You're not a pro and are not presenting yourself as one, therefore, they have no expectation of professional results.
1) You've a nicely cropped photo of the bride/groom at the altar above. It doesn't look overly grainy. I'm not sure if you got that by zooming in or cropping afterward, but either way, it looks okay to me so I'd stick with it.
2) A yellow colour cast isn't exactly unusual for a church. If you want to correct this you can simply play with the white balance temperature to see what looks best. Ignore skin tones. Warm skintones will go down well as people feel they look pale with colder tones. However, the dress must be white. A dress with a yellow tint won't go down well (not that it matters, you've already told them you're not a pro).
3) Don't be 'rushing or hurrying anywhere during the ceremony. If you miss a shot, that's their tough sh*t. I can't stress enough; you're not claiming to be a pro. I understand that you want to get some great shots for yourself, because you want to do a good job, and that's perfectly understandable, but this couple are completely taking advantage of you.
4) If you're giving them copies of the images, resize them down to about 1,000 pixels on the longest side. Don't give them the full size shots. Resizing down will get rid of the grain and makes things look sharper. You've missed the focus in one or two shots above (either that or the image is just very soft due to the high ISO) so a smaller size will make it look sharper.
5) Blindly follow this advice! - If they want the full size shots: Charge them. Don't second guess it or think about it. Charge them.
If they want high-res shots, they want them for a purpose (a nice canvas for the wall, perhaps?). At the end of the day, professional or not, if you do a good enough job that they feel they'd be willing to hang a photo on the wall: Charge them.
6) Don't do a formal/posed group shot. This is something a professional does (and no, I'm not saying don't do it because you're not a pro. The reason I'm saying don't do it is because everyone in the group photo will want to see the photo you've taken, and because you're the one taking it, they'll assume you're a hired pro, regardless of whether you tell them otherwise or not. If your shots go poorly, they'll see a piss poor group photo and then if you ever do decide to go pro down the line, this will work against you. People remember the bad a lot more than they remember the good).
If they ask you to do a big group shot just lightly laugh it off with a 'haha, ah jeez, that wouldn't really be my area at all, I'm afraid. Sorry'. It's jokey, but it gets the point across.
Truth of the matter is, they see you with a 'big camera' and assume you will be as good as, if not better, than the next bloke with a 'big camera'. They've cancelled their photographer because someone told them you took lovely photos and would do it for free.
Only do what you want to do. If you do a group shot, make sure it's because you want to do one. If you do any posed shots with the bride/groom on their own, make sure it's because you want to do them. Don't feel pressured and don't end up running around for them.
Truth be told, one of the reasons I stopped doing free work (I'm a pro) was because I always felt like, when I was working for free, I was shown very little respect. Don't let it happen to you.
Above all else though; enjoy your day. Ultimately, you're a guest. You have no responsibility and shouldn't get worked up or worried about anything like this.
Also, if you're not already doing it; shoot RAW. It'll help with white balance afterwards. Also bring a second memory card if possible. You'd be surprised how 'snap happy' you get during certain jobs.
I wish you the best of luck with it. You seem like a nice guy as you're willing to do it for them.