No, a plug cannot fit a USB 3 front-panel connector into a USB 2 header.
No, a plug cannot fit a USB 3 front-panel connector into a USB 2 header.
You're facing a compatibility challenge between the modern case and your motherboard's USB headers. The 19-pin USB 3.0 connector on the case is designed for USB 3.0 devices, while your motherboard only supports USB 2.0 headers with 9 pins. Generally, USB 3.0 and USB 2.0 are not backward compatible, meaning the case won't work with USB 2.0 devices. You may need a USB adapter or a different case that supports both standards.