Can I exit Turbo Tax Federal Return Preparation until I verify if we received the $1400 Stimulus Check back between March and May 2021 and then pick up where I left off?

Yes.  I have done so multiple times 

Yes you can.  Your tax data is saved as it is entered.  Click on Sign Out in the bottom left corner of the online program screen.

...be sure to save before exiting...

Yes we did recieve the $1400.00.