Savestate is generally a thing where a snapshot of memory is taken, different to a more conventional in game save where the essentials of the game are noted down and then used to repopulate the world when you load back up, and many times a savestate might not work well or have some odd effects when the thing is loaded back in (games tend to refresh things as you leave and enter rooms, or have things loaded from the ROM in advance of actually seeing them on screen).
2 thoughts.
1) Mixing ROM hacks and originals can be tricky. Music and textures might not be so bad but swapped models can make things look odd. The real problems do tend to come when translations (make a save with a custom name using an entirely different text setup and... yeah), game logic changes (new items, different effects) and serious level alterations (save and load back in an altered level layout where the old location is now in somewhere that isn't the level, load back into a base game where you hack gave a key item or saw it not available...) happen so you will probably be OK.
2) See what the Wii has as far as what it uses to tell games apart. For some systems, or flash carts as the case may be, that we play with around here then it can be as easy as changing the serial of a game or name of the file. At that point as far as it is concerned it is an entirely new game. Alternatively if one of the loaders will load Wiiware/VC games rather than using the main menu, or you can make a virtual NAND image with your own profile...
2a) I tend to find siblings lose interest in things like this in fairly short order -- the amount of times friends traded or lent games, sometimes less than stellar ones, around that nobody had even considered in months and then had the "I wanted to play it". If you wanted to sideline (or maybe kick it to an emulator) for a month or two for that to happen then could do something.