1. That's correct, and only needed if you are not going to be managing the switches via the FortiGate. If you manage the switches from the FGT you won't need the advanced license as the FGT will be doing the dynamic routing. The advanced feature license is perpetual, so only need to get once per switch (if you want standalone switches).
2. You'd still need to purchase the FortiCare support for the switches if you want to get support and get replacement switches if one fails. But nothing else in regards to licensing.
3. You can still manage the switches from the CLI using the FGT's GUI. Most things will be done through the FGT, but if there's anything that needs to be configured in the switch you can still access it's CLI.
4. Yes definitely. You just connect the switches and they will automatically form the topology once authorised. Below I've got three switches daisy chained.
5. Once a switch connects to the FGT then it's not longer in standalone mode and the config will be wiped and the switch will reboot.
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