My tus an Howelsedhes!
Fantastic advice from Ken and Bud, Mark it well is to be wise!
Now to capo - Dullness. Do you have a Capo that can squeeze the strings hard enough? I have a Keyser like this.
because it changes quickly, it squeezes hard enough, it looks cool and it has a lifetime garauntee!!!!
Try placing the capo nearer the fret or further from it - do you get a better sound when you do this with what you've got? Are the strings just dull because they need changing?
To the chords getting higher - YES! they do, BUT if you have a piece with some great chords and runs you can find a key that is slap bang in the middle of your best singing range, be that Eb, A# - whatever is good for you and with the capo you can do that with keys with simple chords you know like C - G, Am, C, F; G - G, Em, C, D; etc.
Check out this link courtesy of the venerable Flatpicker 55! This is just C and G (and some DAMMNED fancy left and right hand work) but look where N.B.'s Capo is - is he screeching or singing comfortably?
Can you down tune? Yes of course you can, but you have to be careful how low you go for 2 reasons - less tension can alter negatively the vibration of the string resulting in loss of tone and volume, also the guitar is usually designed and supplied to function at its' best in Concert Tuning - A = 440Hz; if you change the stresses you may need to have the guitar Set-Up to cope with the different tension - the Truss-Rod adjusted and/or the Topnut or Bridge raised or lowered.
I was thinking also that if you are thinking of changing the tuning for different songs mid-gig you will be making problems for yourself unless you are able to interact with your audience at the same time as tuning - like this amazing guy can:
Whatever you decide to do, Buena Suerte ha Chons Da!
I'm sure it'll go fine for you if you relax and enjoy as was said before my rambling.
Oh! Final thoughts: Sort out a set list. Make sure you have everything you could play at that event written down in some sort of order you intend to play in - get requests from bride, groom, best man, etc, etc,.
Also, if you need a music stand (especially if your books enable you to play requests) take it, they are accepted!
Oh, and enjoy yo.... oh yeah, been said!