For question 2, you can
- Add https://github.com/espressif/arduino-esp32/releases/download/3.0.0-alpha3/package_esp32_dev_index.json to the Arduino external URLs, or
- Download the Arduino Alpha 3 Release v3.0.0 based on ESP-IDF v5.1.
Then, you would need to manually update the installed package with some missing parts, which are on the repository but not yet included in the Alpha 3 package. For example in Alpha 3, boards.txt does not list XIAO_ESP32C6, but the repository includes it.
Please take care in using LED_BUILTIN instead of 5 or 13 mentioned in the examples, as the numbers may interfere with the board and the boot-loader. Pressing the BOOT button saved me from a board without Serial.
The WiFi examples work fine. I faced building issues with the Bluetooth examples. I guess the Alpha 3 package still needs to mature.