Going back to the original post and premise
To carry the bomb, Wallis has proposed the 6 engine "Victory" bomber.
in 1940 the proposed bombers were a lacking in engine development which lead to the 6 engine solution with had problems of it's own.
The B-29 worked (barely) because the turbos allowed a sea level 2200hp to make 2200hp at over 25,000 or more importantly to make 2000hp at over 25,000ft at max continuous.
Granted they sucked fuel at a horrendous rate but trying to get non turbo engines to give similar power was not going to work (unknown in 1940/41).
A B-29 at a take-off weight of 120,000lbs could burn 1000 US gallons of fuel just getting to 30,000ft and needed just under 240 miles to do it using max continuous power all the way.
They may not have done that very often, more likely to climb in stages to cool off the engines.
The late/post war 3 speed Griffon offered 2000-2420hp at low altitude but it was down to about 1400hp at 25,000ft for max continuous.
Needing 6 or more 2000lb + engines (and radiators and other accessories) to match the power if the Wright engine installations (with
all of their accessories, like twin turbos and intercoolers) started running the weights up quick.
Using manned defensive guns didn't work well with pressurization. Getting remote control/sighting guns made sealing the crew compartment a lot easier but required more complicated gun mounts. A limited travel tail mount might work.
A large, heavy, high speed bomb is going to be less effected by winds on the way down but dropping from higher altitudes has problems of it's own, like vision.
There is often quite a difference even with rifles between theoretical accuracy and practical accuracy at longer ranges.
Granted part of the idea of an earthquake bomb is that you only have to get it into a area of 4-6 city blocks
But in 1941-42 for the British you do need to find the right city.