OK, so a few basics are needed here as you have some significant gaps in your knowledge which will end up breaking things!
Fist off, you have a stereo - 2 channel amplifier. Each channel will drive a load of 4 Ohms or more. The A&B buttons just add the speakers to the channel, they do not increase the power handling ability. In simple terms, you are trying to connect too many speakers to the amplifier and that's why it is cutting out.
Impedance and Power: Amplifiers will have a stated power output into a stated number of Ohms. IE: 100W into 4 Ohms and 50W into 8 Ohms. This is because of the fundamentals of how electricity works. The 2 simple equations that give you an idea of what's going on are as follows: Watts = Volts x Amps and Amps = Volts divided by Resistance - which in this case we are using as the impedance figure. (For any pedants reading this I am treating impedance as resistance and assuming it is a static figure, but the basic theory is the same as for complex impedance) Power amplifiers multiply current more than voltage. It's usually about 10-20x increase in voltage and closer to 200x for the current.
Therefore, in the case of a 100W / 4Ohm, 50W 8Ohm amplifier with power rails of 50V, the current into 4 Ohms is 2 Amps and 1 A into 8 Ohms. This means that you have double the current available for a 4 Ohm load. So why not add more speakers and make the load 2 Ohms, because the maths would suggest that it would make the amplifier play even louder? The amplifier has a maximum current that its power supply and output stages can supply, so by halving the impedance, you have doubled the current required. This extra current generates heat and the output devices will eventually overheat and either destroy themselves or will start to conduct less and high levels of distortion will result.
It's also important to point out that as sound levels of logarithmic, 100W does not sound twice as loud as 50W. Twice the speakers is not twice the sound pressure level.
You must therefore ensure that you keep the impedance of the connected speakers ABOVE the minimum value that the amplifier can handle. Your highway analogy works for voltage - where a lower impedance will allow more voltage to flow, but where current is concerned, think of it more as a pipe with a pump as the amplifier. If you make that pipe bigger - lower the impedance and increase the (current) flow, if the pump stays the same size it will reach a point where it cannot keep up with increased flow rates and will fail.
You are connecting all your speakers in parallel - lots of pipes all side by side. There's a formula for working out the impedance for parallel connections -https://www.speakerimpedance.co.uk/?act=three_parallel&page=calculator , but for 2 speakers you just divide by 2, so a pair of 8 Ohm speakers becomes 4 Ohms and 4 8 Ohm speakers will become 2 Ohms.
If you want to attach loads of speakers to your amplifier, you can connect them in series - one after the other rather than in parallel. The calculator I've linked to can help you to work out how to connect them to give you the best combination, but if they are all 8 Ohm speakers, connect each pair in series so you end up with 2 x 16 Ohm loads, making 8 Ohms in total.
The next questions is why the hell would you want to??? Multiple speakers just sound horrible. The HFs will comb so that the "system" will sound different as you move around the room, the bass will be very lumpy for the same reason and stereo imaging will be vastly reduced.
If this is all about ultimate level and not quality, you bought the wrong amplifier. You would do better with a big class D PA amplifier that can tolerate loads of 2 Ohms and simply stack the boxes up and revel in all the waffly, inaccurate glory...
If you want a surround sound experience, buy an AVR with multiple amplifier channels and then you can safely attach all the speakers and use the DSP on the AVR to add surround effects. Using a stereo amplifier with lots of speakers is hardly ever accepted practice.