Welcome to the forum.
There are a few problems with your idea.
Unfortunately there aren't any quick release axles for hub motors. They require a more solid mounting than a skewer can provide. Even the tiniest motors make enough twisting force to snap the dropouts and cause a wheel loss if not mounted properly.
Controllers need air for cooling. mounting them in a bag can damage or destroy them, and is potential a fire hazard if they get hot enough. Generally they stay cool to the touch, but can get very hot in normal operation. Controllers also don't like to be bounced around in a bag. they need to be mounted solidly to the bike frame
There are no clip on throttles. Technically, one could be made by replacing the bolts with thumb screws.
"Waterproof" is relative. The best are more like water resistant. it matters more on how you mount the connections
The idea of a quick removal kit comes up every few months. Unfortunately, its not as good idea in practice as it would be in theory. Luckily, there are alternatives, depending on your needs..
The easiest is to mount a tiny Andana Q100. its 1.5KG, and freewheels when not in use so it causes no drag on the bike. the controller is small and light and can also be mounted out of the way. that also means you can leave the throttle in place. That also allows you to mount the wires much better, assuring you can properly waterproof the connectors.
THe battery is removable, and is the bulk of the weight anyway, so when you don't want an Ebike, you just pop off the battery and ride. the weight of the q100 system isn't much different than the weight of a good rack system, and won't affect your performance.
As for the battery size, thats tough. At 30MPH a 25KM ride would need 15AH of capacity. you would often get further, but batteries lose capacity over time.sometimes 10-20% per year regardless of use. And riding in headwinds, up hills, or deep cold takes far more power than a windless spring day on a flat smooth road. And lithium batteries don't like to be drained all the way very often.
7AH might get you 25KM under perfect conditions. But in the real world, you're going to need double that.
As for ebrakes, you may need them for legal reasons, but with a geared hub motor, they serve no real purpose. their job is to cut motor power if you forget to let go of the throttle and grab the brake. Motorcycles don't have that feature. cars don't have that feature.
As for pedelec systems. Most kits have that as an option, but a few kits are designed to need to be pedaled in order to work. make sure the kit you order is set up for TAG (twist and go), or Immediate start operation.
