Fermentation - Can it happen in 1 night?

Yes, it's possible. Particularly at 75F, which is really too warm for most yeast strains. Or rather, the yeast love that temperature, but you'll most likely get elevated esters and (possibly) fusels in you beer. BTW, hydrometers don't really measure ABV. You want to be looking at the specific gravity scale.

Edit: just because you're not seeing visible signs of fermentation at this point doesn't mean that it was finished, or that it is not continuing. Wait a week and take a gravity reading. Then take another 2-3 days later. If it hasn't moved and is in the right neighborhood, then attenuation is done.

 

It depends on your yeast strain, it should say in the instructions or you can google the yeast strain to see the optimal temp.
Most ales say from 55F - 70F give or take, and you generally wanna get it close to the colder side to prevent unwanted yeast character from coming through.

But vike said above yeast LOVE hotter temps and will work faster when they are in a hotter environment. But this will produce unwanted flavors and more fusel alcohol which is linked to headaches and a hot alcoholic taste.

If your fermentation is stuck you can attempt to "rouse" the yeast by swirling it around, just be careful not to splash and oxidate the hell out of your wort.

Not sure about throwing more yeast in it, someone smarter than me will probably know that answer.

 

Yes, if you get a stuck fermentation due to an unhealthy/inadequate pitch of yeast, you can pitch more. But in your case, there's nothing to suggest that it's stuck. This is more than likely a non-problem. So again... wait several days, and then take a gravity reading.