Combining speaker outputs to one secondary amplifier
With a bit more searching I think I may have come across the solutions.

https://sound-au.com/articles/audio-mixing.htm

It looks like, for a simple combination of inputs a simple resistive circuit as shown in the question with simple mixing of the outputs is fine, if gain control is required on the individual inputs then it is better to use a virtual earth mixer is best.... so I think what I propose should work.

I'll leave the question open for a couple more days to see if anyone can add any further inputs.

update:
Yes that solved it. I have three pots dividing the voltage across the three speakers (left, right, centre) at the speaker outs on the receiver. The outputs of the pots are combined (mixed) into one input for the mid-woofer crossover and then through to the amp... pretty much perfect now... one day (when we get a bigger home) ill be able to get proper speakers and get the best sound out but this is a great workaround for a small house.