Use x86_64 since you have a 64 bit CPU. Use a "combined" image, see below. The choice of ext4 or squashfs is how the file systems are organized, either one will work. The choice of EFI or not depends on whether your virtual BIOS supports EFI booting.
OpenWrt does not have an installer like most Linux OS. It is distributed as a direct disk image. When uncompressed with gzip and written block by block (raw) to a virtual disk, there will be a partition table and two partitions. Boot this disk as a legacy OS.