I think it is a fair assumption that they'll need Mari's power to escape from wherever they end up. Alice is strong and has mastery over most common spells, but that did not look like a common spell. Mari seems to be able to use some very high level magic (conditionally, maybe? it was activated by strong emotion in the case of the frog I think).
Mari seems like she's one of those 'very powerful but has no real control over those powers' characters. Getting home for them from wherever they end up could be as simple as Mari feeling homesick and wishing to go back to the school (though I doubt either of them will realize that).
You know, wouldn't it be a neat twist if magical ability with a full range of emotions was insanely powerful, and because of this, the witches have been purposefully left this way? The dead hearts and inability to use magic while in love could be a two-part system to limit witches from reaching their full potential and outstripping humanity. We know that the witches pledged fealty to different nations, so it wouldn't surprise me if this was a condition of their loyalty.
Mari in that hypothesis would be a witch who somehow was left out of this, discovered by the academy, and now they want to learn how she's so powerful in order to eclipse their masters.
Main problem with my theory is that they have to be unaware of the downsides of the dead heart thing for this to be possible. Alice did seem to know about both restrictions, but maybe she was taught that this was just a fact of nature with no exceptions.