Axial frequency response of the 210 and 410 will be almost identical. It would favor the 410 with enough voltage for the 210 to suffer from power compression both thermal and mechanical, but that's a different question than what the OP asked.
True, the mids will sound louder off-axis.
Not quite. In the OPs scenario the voltage to each cab is the same. In the case of the 210 the displacement (T/S spec Vd) is doubled. When you double displacement with the same voltage to each driver, as is the case with parallel wiring, you get a 6dB increase in sensitivity. Doubling Vd again with the 410 would get you another 6dB with parallel wiring, but with series/parallel wiring the voltage to the individual drivers is halved, which reduces sensitivity by 6dB for a net zero gain over the 210. To realize the 6dB higher maximum SPL from the 410 compared to the 210 you'd have to turn up the volume to double the voltage output, which then results in all four drivers receiving the same voltage as those in the 210.
The takeaway here is that it's not the power or cone area that counts. It's the voltage and the driver displacement.