Using Time Capsule with an existing network

I'm wondering if the Time Capsule will be right for my home and current setup;

At the moment, my main router (with wifi) is on the ground floor. The middle floor has an ethernet switch (connected to the main router) and the top floor has an ethernet switch (connected to the switch on the middle floor).

If I connected the Time Capsule to the switch on the top floor, could I use it as a wifi hotspot? (I barely get a signal on the top floor, so it'd be great if I could)

Would I be able to access the Time Capsule backup from any floor? Even if I was connected to the main routers' wifi and not the Time Capsule wifi?

Could I connect devices by ethernet to the Time Capsule? If I do, could that device connect to all other devices on the network or just devices on the Time Capsule?

What security do I have available to stop other people on the network from accessing my backup on the Time Capsule and the settings for the Time Capsule?

QA Apple Time Capsule

QA Apple Time Capsule

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