What is a good way to move my MacBook life to a temporary computer, and back to primary computer?
Make a complete copy of the primary computer's hard drive using Carbon Copy Cloner to an external SSD / hard drive. Boot the secondary computer from this new disk (turn off computer, connect drive, hold option when restarting to select it as the boot disk). Booted from the external drive, the secondary computer will be just like the primary one now, same files and configuration.

When the primary computer comes back, use CCC again to copy the data from the external drive back to its internal hard drive.