So I finished my first run and then continued from right before the final scene where they drain the lake, and I went searching for items. 
Now I have every item in the game except for the ones that are in the far eastern section of the map Right under the lake (which is blocked by the explosion, as far as I can tell) 
 
So is there any way to get those items without restarting?

Fuck, Can a mod put this in Shadow complex for me?

restart.

You can get any missing items even after you triggered the final encounter at the lake.  There is a path that you can go through underneath the lake.  Check the map on where to go.