So ... if I understand correctly your game crashes with or without the trainer which makes it a game issue.
Have you verified the integrity of the game cache or completely uninstalled and installed again?
As to Alt + Tab, some other things to try.
Instead try using Ctrl + Escape.
For that and/or crashing in general, are you using FullScreen? If so, try using Borderless Windowed mode instead.
If you're using Borderless Windowed mode now, try using FullScreen.
You might also try turning off the Steam Overlay. There is a setting for it under Steam/Settings/In Game at the top left side of the Steam Browser.
You could also even use the Steam Overlay in place of Alt + Tab or Ctrl + Esc.
Try changing your outfit back the default one - was already asked and answered a couple of times in this thread.