That's the one, my mistake. Also, it really is incredible the amount of things various characters remember. Keeping that gross bandage you have at the beginning on and never taking it off even triggers some dialogue.
Typically with Steam Cloud games, if your files are different, it will ask you if you want to use the files on the computer or the ones in the cloud. I never got that message despite my save file shenanigans, so I assume that it doesn't really work with Steam Cloud. If you really want to be safe, you could just do the thing RedLuigi said and reset before you get to the barrier (you have to make a choice, try quitting before the choice) and look up the rest if you're especially curious.
First of all, no they don't. The change is only at the end of pacifist runs. Second of all, delete the files in the AppData\Local\UNDERTALE folder. Problem solved. Basically what I did was I kept my old save and replaced the genocide save. Apparently that near-blank achievement file is the one you need to delete, but I wanted to be safe.
Some of the boss deaths in this game are pretty brutal, even in neutral runs, and it's only enhanced by the fact that you caused it. A lot of people say they're just going to watch the genocide run, but you don't get the real sense of killing your friends unless you do it yourself the first time.
The Toriel fight is basically teaching the player that just ACTing isn't always the solution. I made sure I never touched the FIGHT command for the entire game, so I just tried the only thing left to do. This is how I learned how to beat Undyne as well.
Technically Steven did it, but that didn't end well. All gems can shapeshift, but most don't do it if they don't need to, and there isn't much need for it anyway. Amethyst just likes doing it because she finds it fun, I guess.
I Genocide'd twice to do the fight again. Fortunately, killing's just one of those things that gets easier the more you do it. Knowing how to vibrate against walls also helps. I've also been poking around some unused rooms using Cheat Engine, but haven't found anything really worth noting. It is a fun way to break the game, though.