1
For the Bitlocker Key problem:
If your drive is encrypted with Bitlocker, the only place the Bitlocker Recovery key is automatically stored is on the associated Microsoft account on the link below, be sure to check any Microsoft account that may have been used on the PC and also any work or school account that may be linked to the PC.
https://account.microsoft.com/devices/recoverykey
Are you able to find the Bitlocker Recovery Key on any account that may have been used or linked to the laptop?
2
For the UAC prompt problem.
If the Yes button is missing, that usually indicates your user account is not an administrator account on the PC, did you make any changes to your user account before that problem occurred.
Also, if you click your Start Button, then just type netplwiz and press Enter, does that utility open or are you blocked by an Administrator.