Unable to mount partition
You need to enter the command given below.

diskutil info disk0s2 | grep -e Size -e Offset


You should see output similar to what is shown below.

   Partition Offset: 314597376 Bytes (76806 4096-Byte-Device-Blocks)
   Disk Size: 351.0 GB (351027175424 Bytes) (exactly 685599952 512-Byte-Units)
   Device Block Size: 4096 Bytes


You need to determine the offset and size in units of device blocks. In your case, the offset is 76806 Blocks and the size is 200000118784 Bytes/4096 Bytes per Block=85699994 Blocks.

Next, you need to boot to macOS Recovery. Basically, restart the Mac and hold down the ⌘+R key combination until macOS Recovery starts to boot.

From the menu bar, open a Terminal application window. Enter the following commands.

gpt -f remove -i 2 disk0
gpt -f add -i 2 -b 76806 -s 85699994 -t apfs disk0


Removing the APFS Container with the Untitled and Untitled - Data Volumes

The following commands can be use to remove this container. Here I assume you have booted from the macOS stored in the other container.

sudo diskutil apfs deletecontainer disk0s3
sudo diskutil erasevolume free none disk0s3


An alternative and perhaps safer way to remove this container would be to first boot to macOS Recovery over the Internet, a macOS installation flash drive or an external drive with macOS installed. Next, enter the following commands.

diskutil unmountdisk disk0
gpt remove -i 3 /dev/disk0.


Next, reboot back to macOS.

When finished, the drive will contain 150 GB of free space. This free space can be added to the remaining APFS container by entering the command given below.

sudo diskutil apfs resizeContainer disk0s2 0