Some thoughts:
After a fast reading of the datasheet, it appears that this sensor can be read in SPI 4 wires by default or 3 wires.
1/
If you are using the default SPI mode 4 wires (Pin 11 to Gnd, page 106), I guess connections should be:
SDI to MOSI, SDO to MISO, CSM to CSS, SCK to SCK, Gnd to Gnd. Is that your wiring ?
2/
I noticed (datasheet page 18) that :
Accel_Data = (uint16_t) (MSB << 6) // Bits 13 to 6
| (uint16_t)(LSB & 0b00111111); // Bits 6 to 0, remove flag bits
which is not what I can see in your code.