I had the same problem, the memory could not be read error, both with hammer and cs:s. After miles of research on it, there's only two solutions I found that work for it.
a. If you have more than one module of RAM, take them out and switch slots.
b. Complete Reinstallation of Windows and Reformat of Hard-Drive.
This has been a HUGE problem since some of the latest steam updates have come through.
As for the entities, solo is right, it's an update, and there's no fix. The only way to fix it is to find an older version of hammer 4 before the update, but for the time being, you'll have to memorize the symbols and try to remember what you placed and where :-/.
Sry muzzleflash, if you find other solutions to these problems pls do tell. I'm just giving you my experience with these errors so far.
Edit: I should add, I don't know what update caused it or where to find earlier versions.