Here's a quick sketch of my take on this from looking at the map.
The Combs and Toddbrook reservoirs (dark green) feeders (yellow) fill the High Peak Canal (canals use a lot of water when boats go through the locks, about 30 tonnes in our local canal as the locks hold two 70ft narrow boats).
Because the canals follow the contour and the Goyt is lower than the canal after Whalley bridge it has to be filled from Toddbrook and Combs reservoirs. Eggs has said it is also filled via the Macclesfield canal and the Bosley reservoir which is 25km from the junction with the high peak canal which starts in Whalley bridge about 9km south of the junction.
The Fernlee and Errwood reservoirs do not seem to feed the canal but presumably also have the function of controlling flow on the Goyt. The trouble is the maps only show open water so I do not know what any of the reservoirs also serve by underground pipes.
So the only ways out of the Toddbrook reservoir are via the feeder to the High Peak canal, the weir at the northern tip and then the overflow that runs curving clockwise in front of the dam and thence into the Goyt and the spillway over the top of the dam which meets the above overflow.
Any excess that ends up in the High peak canal ( shown red) flows over a weir and into the Goyt (shown light green) near the start of the canal at Whalley bridge.
As the Toddbrook reservoir was full, and following very heavy rain, all the excess ended up in the Goyt, which seems to have coped with it, as I said in an earlier post there was the possibility of holding back water at the Fernlee and Errwood reservoirs plus the Combs reservoir if the Goyt could not handle the excess from the Toddbrook system.
So it looks like everything worked as planned except the spillway was damaged when it was over-topped with an unprecedented flow. I can see the spillway was poorly constructed or maintained for the flow that came over it, Eggs asserts this was predictable and attempt should have been made earlier to lower the level of the Toddbrook reservoir via the overflow into the Goyt if I infer his post correctly.
My personal guess is that there wasn't much of a void below the spillway slabs prior to it being over-topped and the damage was all done during the surge because the joints between the slabs had deteriorated.