Questions about Homematic IP Smoke detector

Hello Yes, a single detector works, but you need the app to register the smoke detector, you also need the AP, if you want a single detector without an app, I would not recommend this variant, then even a cheap detector that simply beeps when it detects smoke is sufficient. The homematic systems are designed for a network of detectors, which is why they are more expensive than the simple cheap detectors.

yes, see the description on the manufacturer's page: https://www.eq-3.de/produkte/homematic/detail/homematic-funk-rauchwarnmelder.html

For linking with Google Assistant, you need the central control unit or at least the AP and the Homematic IP app. Take the starter kit with 3 detectors and the AP - the price is almost the same as three individual smoke detectors: Homematic IP Starter Set Smoke Alarm