Is Audition's History the only way to reverse Fade In and Fade Out?

Waveform view is destructive - any changes you make to a file, and subsequently save, will be permanent, as it's the actual file you've edited. Multitrack view is completely non-destructive - think of it as a giant file player, if you like. What gets changed there is just the information about exactly what you're playing out of a file, and this can be very selective, and also repetitive if you want. And you can add and remove effects - because they aren't added to the file as such, but the playback of it. So if you put a fade at the start or end of a file in multitrack view, you can remove it again with impunity.

The only time anything from multitrack becomes permanent is when you mix it down, or export it to another file. That new file becomes your permanent copy. But hey, if you don't like it, then change the multitrack mix and make another! All the information for multitrack sessions is stored in a session file, and that's all you'll be altering - you don't touch the original files at all.