When photographing wedding formals, the medium format camera should
be on a tripod.

You are on the right track...., what is this St E2 transmitter?  is it an
infrared?  I tried that once, and yeah, in a large space it got lost.   a
radio tranny will cost about $300 for both ends, well worth it.

one problem is that a 27 inch umbrella is basically worthless, especially at
the distances when lighting a group in a large space,  at 20 feet or so, how
much bigger is a 27 inch light source over the basic flash head itself,
think in terms of arc, the number of degree's in a circle, not much
difference at all, and you are risking a breeze catching it and knocking the
whole thing down, like when the air conditioner kicks on.

I used to use mini dome softboxes, halos etc.  and they are ok for close in
work but then you are maneuvering bulky things around at the top of a pole.

a radio transmitter sync will remove sync problems, usually, what I've done
on occasion was to take a visual slave eye, and if that didn't work, take
that slave eye and plug it into an extension cord and dangle the damn thing
in front of my on camera flash, (weird I know but you gotta do whatcha gotta
do sometimes)

you really should use a tripod, it really does help, not only with
sharpness, with giving you free hands, helping you keep other cameras back
behind your work space, but, it would allow you to use the ambient light as
an extra light source.  You've seen the great value in 3 dimensional
lighting, another step up is to shutter drag so the background light level
builds up to one stop under your key light.  this gives you extra detail in
the background, make the negs much easier to print as there is more for the
lab's analyzers to look at, (otherwise there is a lot of clear space around
a very dense spot in the middle, that white dress and that is hard to judge)
a slow shutter speed would have allowed the ambient light to provide some
fill, which I just noticed from your post that you did not use, yikes, and
with such a small key light source.

time to rethink, key and fill with specular light sources, slow shutter to
pick up ambient levels, a very nicely exposed neg.

I do not think a wide angle is a good idea for groups, the problems of wide
angle distortion on the folks on the side can be obvious some times, and as
you noticed, very uneven lighting becomes really problematic.  I used a
slightly long lens, (110 on my 645) and this gave me a chance to move back,
keep the others way back, and give more even lighting, and more even
perspectives on the folks on the ends.

yes a hood would have helped, I love hoods, used a big bellows on mine, with
a vignette to knock the edges down a stop or so.  loved the effect.  and it
helps keep stray light out, and you know what, it just looks so cool...

the reply is echoed to the z-prophoto mailing list at yahoogroups.com where
there are a lot of posts about double or key/fill lighting at weddings.