Hi Viveroth. You mentioned that you live in Poland, just want you to consider that you need a 220V electrical connection for the bulbs in order to work properly.
About the questions:
1) Yes you can
1a) If you want, you can activate the "Select a Default State" option or "Save the last state" (of the bulb before you turned the off)
1)(Again?) According to the developers if you select the option "Save the last state" it may reduce the life time of the bulb.
2) If you want the bulbs to turn on with the same color as you turned them off and want to avoid the risk of to reducing the lifetime of the bulbs, I think you should.
3) I think it doesnt matter as long as the switch have yeelight (xiaomi gateway) support.
4) Yes the Mi Home Gateway (or xiaomi gateway)
5) In my case, I always preefer the app or google assistant/google home
6) Of course!