Why not just use a back cover to hold all the modules ?
I think that the current solution to the module layout is inefficient.
The area of the partition
(marked in yellow) itself equals to at least another small module, IMO, a phone can't have so much 'dead' space and the more features a phone has the better. The partition area and the magnets also add weight to the device which could otherwise be used for more features.
Secondly, instead of 'wasting time' on fancy new features that we've never seen before, they should focus on getting the phone and modules to work together and release it for consumers for now and then later focus on these bleeding edge tech.
Thirdly, if water will spill on the back they could potentially sip through the cracks and get to the connection causing a short circuit and oxidation.
Lastly, the more expensive parts and the time to assemble them means the consumers would have to pay more. The whole point of this project is to make smartphones more accessible by making this significantly cheaper than current smartphones.
I think a back cover with clips or some mechanical locking mechanism to hold all the modules would be a much better solution. It will probably make switching modules a bit more complicated but it shouldn't be harder than changing the battery on a current smartphone (like in the Galaxy S line), in Ara's case it would be like changing several batteries.
TL;DR: I think a back cover to hold all the modules in place would be a much better solution because of the 'dead space' of the partition, weight, water protection and cost.