Maybe I'm misunderstanding, but it sounds like the main speaker covers about 40' back to the delay speaker which then covers 60' back to the rear wall. If both speakers are aimed down at a 45 degree angle then unless they are mounted very high it seems that the vertical axis of both speakers would be aimed well forward of the furthest listener in their coverage area, in fact possibly even in the front half of the area they cover. This usually results in less than optimal coverage and a drop off at the furthest listeners. That would definitely make the rear of the room a poor location to mix from and it might even mean that your proposed mic position at the rear of the main speaker coverage might be hearing something quite a bit down in level from some other areas in the seating.
With a 90 degree horizontal coverage from speakers mounted at any significant height in a room only 30' wide, it at least seems that there would be much of the speaker's energy hitting the side walls rather than the congregation directly, thus resulting in high level early reflections.
My concern is that these two in combination, reduced direct level and high levels of indirect energy, would often result in intelligibility concerns. If you auditioned the speakers with them mounted in a way that represents the eventual system configuration and using speech as a source they sounded good and easy to understand throughout the intended area of coverage, then perhaps there is something specific to this application that is preventing such issues. However, it would generally be something to consider.