So, I have tested the background context menu issue on four different computers with three different graphic cards and with tested RAM (memtest, several hours). When the background context menu is open and I move the cursor into the menu, it changes the underlined characters on all tested computers with all tested themes. I also tried Mint 14 MATE without this issue, so it is a bug in Mint 14 Cinnamon.
I haven't enough time to test the window corner issue on all computers, but I noticed it on two computers with tested RAM and with Gallium 0.4 on ATI RV350, so I would like to ask if anybody with older ATI noticed it, too. Mint 14 MATE on the same computers works without problems.