jah.
i really think before "going live" as it were you need to do some test runs
with the group there and running biab on the mac live.
and see in a real situation via test runs any probs that might occur.
i dont care whether pc or mac or auntie flo's computing company..lol.
any computer can glitch live. a hard dtive can go south,
the power feeding the puter can go south etc etc.
so my advice is plan for the worst.
and have a plan to over come probs.
in a highly computerised situation anything can go wrong.
some touring acts go as far as useing redundant puter set ups so if one goes
winky another carries on in real time.
we are talking here a real time situation...and lots of things can go wrong in
real time situations.
computer architecture whether mac or pc has not yet reached
the level of sophistication of built in redundancy.
the motherboard goes south and your up the kyber pass..lol.
i know what you want jah...to be able to change your mind
"on the fly" during a song...just be aware , and i dont care the application..
ableton, or uncle freds handy dandy live performance music software..
it will fall down if the underlying puter goes south.
if your really serious bout this approach then..i would look at everything that could possibly cause failure...eg make sure the power is really good feeding the mac,
and make sure the mac itself is fully tested under stress , and dont forget the hard drivs used in the mac. i would make sure the drives are very good quality with a good mtbf rate etc etc.
prolly a premium drive should be used. but even they can go south.
sorry to be the devils advocate here (no pun intended...i'm very religious myself.)
but just suggesting you plan for the worst.
so the congregation dont have a good laugh if suddenly the set up falls down.
which has happened in live situations.

mebe do a survey also of what other worship groups have found
in the way of solutions to what your trying to do.
ive seen some pretty fancy worship set ups in the past myself.
but as always fancy set ups cost big money.
which is why i suggested a least cost mp3 route.
god bless. (sincerely mate.)

retired puter engr....powertracks on amd......NICE !
"what is the black art of audio engineering ?"
my silly songs...motagator.com/bmanning
see my tips in the tips section.