Here’s a thought. Given your experience level and lack of practice with the tools you have, you have to keep things as simple as possible. You’re not being paid to produce technical masterpieces, you’re being asked by your friends to help preserve memories. So.. some basic pointers to increase your chance of success – don’t try to get fancy.
First, use a low ISO value (200 would be good). The lower the ISO, the lower the noise in the shadow areas.
In your custom functions, turn on the “highlights priority” setting (c.F6, pg. 186 of your manual). This helps reduce the chances of your highlights getting “blown”.
Other menu settings: set your “Quality selection” to RAW + Jpeg.(shooting menu 1 (red)) It takes more space on the memory card and is slower, but allows you to get photos (JPEG) without going through the post-processing required of RAW. It also allows you to use RAW to correct some of the shooting errors and setting errors that you can make (such as setting the wrong colour balance).
Consider setting the review time (same menu) to “Hold”. Then you can look at your pictures and decide if you like what you see. As soon as you start pressing the shutter button, the back screen will revert to the shooting screen.
I’d set the photo display to show the three-channel histogram. This will be valuable in reviewing your shots. If you have too much on the right, you’ve got blown highlights. If the histogram shows most of the pixels in the middle range, you’ve got a good exposure. If you have lots of pixels at both the left and right sides of the histogram, then the scene you’re shooting is exceeding your camera’s ability to capture (too much contrast), and you are getting both blown highlights and featureless black shadows. You can’t post-process these shots into anything good.
While you’re still setting menu items, I’d recommend you turn the “Beep” off as it can be very annoying to hear the camera beep all the time while you’re focusing, shooting, etc.
Set your white balance to whatever your primary light source will be. If you change from outside to inside, remember to change your white balance. It won’t affect your RAW files, but it will be encoded into your JPEG files.
Consider shooting primarily in Av mode, which is where you set the aperture and your camera sets the shutter speed.
Use low aperture values (such as F2.8 or F4.0) to create a shallow depth of field when you need to isolate your subject from the background. Use aperture values of f/8 or f/11 for group shots and situations where you need to get everything in focus.
If at all possible, keep the sun behind you (well over one or the other shoulder would be better to avoid squints from your subjects) – that way the tonal range of the scene is more or less even. If you have to get into back-lighted situations, use either a fill flash (which you said you knew how to use), or select the spot-meter metering mode and use the spot-meter reading to set the exposure off your subject(s) faces.
Changing lenses can be both dangerous (to your equipment), annoying (to other people), and time-consuming (to you, causing you to miss important stuff). So I suggest you start with the wide-angle zoom, and work with that. Experienced photographers usually don’t change lenses – they have a body for each lens that they plan to use. You don’t have that option as you have only one camera body, but there are good reasons (some stated above), why you DON’T want to change lenses during an event.
As Derrel and others have already pointed out, getting “professional” results takes a lot of time (experience), practice, good equipment, intimate knowledge of lighting, and good post-processing skills. Maybe they make it look easy, but it isn’t. You don’t have the time to acquire all of that. So... do the best you can, remember your limitations, keep it simple. Doing photography in a high-stress situation like an engagement is like participating in a race – you better have the fundamentals ingrained into you and instinctive. You don’t have that level of practice, so to increase your chance of success, KEEP IT SIMPLE. Reduce the number of decisions you have to make, and you increase the chances that the decisions you make will be good ones.
Oh, and get at least two memory cards of 8GB or more. They will hold up to 250 RAW+Jpeg each. If you think you'll be shooting more, get more cards. Also, get a spare battery for your camera and make sure BOTH are charged. If you can arrange it, get (buy, borrow, rent) a grip for your camera - then you can use AA batteries to shoot.