Stuck in present day cave without all the power sources

Nope, the game will force you every time you have to get one.

But you can go and put them in their socket at anytime you want (after the related present mission). I didn't do it before they ASKED me to do it in the main game.

The only way I've noticed it works for me is when I have a enemy that you can usually counter with B, and then hit X to do an assassination kill.

When the other guards are about to fire, and you are near one of the easy guards that I described above, look at the HUD on the bottom right, at certain times you will see the A button show different text for you to grab someone for a shield.