Hi
" So with any sort of Fx applied to Video, you do A to B to get a result,
But there is no simple way to undo Fx from a finished product, no B to A. "
That's more or less it. Once you have exported your work it becomes a single video and the effects can't be reversed in the exported version. After exporting VP will still have your project displayed and you could step back to an earlier point using the redo tab. Of course if you haven't retained your work in some way it will be lost. The "Master" you refer to would be an easily created small text file with all the info needed to re-assemble your work... a .vpj file. So, to clarify the save options for this if you don't already know them...
VP will allow you to save your work at any point, either with or without all the component clips. this will include a .vpj file which are instructions that tells VP how to reconstruct your project.
If you want to move all your work to another PC or upload it somewhere use the option Save Portable Project As. This will create a dated folder and copy all the clips to it as well as a .vpj.file. This folder can then be transferred to the new machine or uploaded to a web site like Dropbox etc. Since the clips are present in the folder the .vpj file can be used by VP to recreate the project.
If you just want to save your work at a particular point so as not to lose stage you have reached, you should use Save Project File As. This does nothing with the clips which stay where they are, but simply saves a vpj file with the name you supply. e.g. "Project 1 PointA". After a bit more editing you can repeat the Save Project File As with a different name e.g. "Project 1 PointB" and so on. This process creates an additional .vpj file each of which will enable VP to reconstruct your project at the point at which you saved it.
If you just use the Save Project File option, you supply the name the first time around (or use the default Untitled.vpj ) but subsequent saves using this option overwrite the old file using the same name There is no opportunity to enter a new name so previous saves will be lost and you will only retain the last save . That's why I generally use Save Project File As.
The last point is that provided you haven't deleted or moved any of your clips VP will recreate your project from the vpj file (even if you have emptied the cache.) If any clips are not present on your PC or you have moved them to another folder, you will get a message to that effect and a prompt to locate them.
Hope this helps
Nat