Where should I place the adverb?

Solution 1:

I believe that potentially would be considered an adverb of certainty in this context. The adverb should be placed after the first auxiliary verb could, so the answer is number three from your list:

It could potentially be moved back to where it was.

Solution 2:

There are three positions for adverbs in a sentence, front, middle and end. Front and end mean what they say. Middle can be either after the first auxiliary verb, after be as a finite verb, or before any other finite verb if there is no auxiliary verb. Linking, viewpoint and stance adverbs generally occur at the front; degree, frequency and adding or limiting adverbs generally occur in the middle; and manner, place and time adverbs generally occur at the end.

The difficulty in your example lies in deciding what kind of adverb potentially is. It doesn’t fit readily into any of the above categories. It could be seen as a stance or viewpoint adverb, in which case it would go at the front, as in (1). However, I’d be inclined to see it as a limiting adverb. It would then go in the middle, as in (3), where it appears after the first auxiliary could.