A: It works with all three platforms. I use the Goggle platform when I decide to use voice. Use these for the stairs, the kitchen, livingroom, bedroom, hallway, front porch, front yard light, back door light, and side of house light where my driveway leads into the garage. I personally would buy them anytime over anything else that is out there unless something else comes out that is better. But I doubt that will happen!
A: Yes the switch works both with Siri and google
A: both siri and KASA (TP-Link) are supported for integration with IFTTT . IFTTT is free, and it allows you to connect dissimilar systems. It can be found here https://ifttt.com/ You will have to explore what features currently exist for the two technologies. If you have development skills, you can also create your own IFTTT integrations.
A: Yes you must have a neutral wire for the switch to work.
A: Yes. A neutral wire is required. Without a neutral it would turn off but you would have to manually flip the switch to turn it back on. Without power it may even require a reset everytime you turn it off. If you don't have a neutral wire, I would not recommend this switch