By 'multi-directional' mic, I assume you mean
omnidirectional (although that could also define bipolar/
figure 8 or
hypercardioid) while the uni-directional is probably
cardioid.
What is the purpose of the rear speakers? Since you indicated that they are getting the same signal as the front speakers, my first thought is that they are probably causing more problems than they are providing any benefit. And getting the front speakers to be aimed at the audience while minimizing the sound hitting the walls would probably help.
EQ may help, but there are certain things that may be related to your room and
system that equalization cannot really
address. For example, you might reduce the energy driving a particular room mode using an EQ but you can't actually fix the problem that way. And as Eboy noted, too often you end up with a conflict between the equalization necessary to correct problems and that equalization negatively impacting the sound quality. This is especially true with wide band, such as octave, equalizers since each band affects a fairly wide range of frequencies and problem frequencies often fall between two bands.
Is this a traditional mass with all vocals and speech with no
stage monitoring or is it a contemporary mass with live music, instruments on
stage,
stage monitors and higher sound levels? How are the mics being used both functionally (lead vocal, choir,
instrument mic,
etc.) and physically (handheld, stand, overhead and so on)?
How big is the room and the audience? What are the room finishes? Do you get an echo off the rear wall even with the sound
system off?