Don't claim to be an expert on Audacity but recently been experimenting with it.
Once you have a recorded project open, clicking onto the Effects tab displays that long list of possible effects (about 15 of them - from Amplify to Wah-wah). So any of the effects can be activated (unless perhaps you have a lite version where you are locked out of some?). It confused me for a while - the effects list are greyed-out if you don't have a live project open.
Once you've used an effect, you have to save the project again. Having saved, you can't go backwards unless you 'save as'. That's the way I test it - keeping 2 versions and compare. But keeping more than two versions is a no-no because of the humungous amount of disk space the programme uses to record, until one downloads the output to .wav or whatever.
Can't answer your 2nd question.