ok, there are no 1/4'' inputs on the back of the ua5. there are only coax, optical, and rca inputs on the back so unless the venue has an AD with a spare digi outt you will need to run the board feed into the rca inputs.
now if your ua5's preamps are stock, you can run 1/4" on the inputs in the front as they are designed for both xlr(loZ) and 1/4"(hiZ). if the pre's have been oade modded then the 1/4" inputs on the front have been disabled. the problem is, if you go this route, then you will need some other mixer for xlr inputs from the mics, or xlr>rca transfromers.
So i recomend going mics, xlr in the front, and SBD, rca in the back. How you adjust the percentages depends, to me, on the sound of the room and where you are located. if i am in a good sounding room and have long enough cables to run the board feed to the sweet spot, then, like flask said, i put more audience into the mix(unless crowd is nuts). if i'm off center and it sounds just ok by the board, where i'm set up, then i go 50/50. if it sounds like shit where my mics are, then mostly board into the mix. so "be the judge" when you get there.
The delay, IMHO, can start to be slightly noticeable at distances past ~50+ft. others may have different opinions since we are talking milliseconds of delay here. the more board mix you have in the mix, the less audible the delay will be. I think the delay is most important for the people gonig for a 50/50 matrix where there mics are far away(think big bands' archivists). you will most likely be fine without a delay.
yikes that was long; hope my small diatribe helped! sounds like you'll be fine. there are definitely some board members on here who own and/or have used delay units in the field that can definitely provide better info for you on that topic. good luck using the ua5 for matrixing. once you get the level adjusting down you will be making some sick tapes!
peace,
Brian