In a thought experiment, you can assume anything, however unrealistic, as long as it's logically consistent with the theory on which you're basing it. Assuming away stray electrons is therefore OK in this particular thought experiment, since the assumption of a universe that would provide an endless completely obstacle-free path would still be consistent with special relativity. In fact, among the standard conventions for discussing thought experiments is not to bring up objections about such things, since it's presumed that the author is intentionally assuming them away to make a more essential point about something else.

In contrast, introducing objects that move at exactly the speed c into a thought experiment based on special relativity results in a logical inconsistency. It's the same mistake as if you assumed that Peano axioms hold and then started talking about a natural number such that zero is its successor. Since the very definition of such an object involves a logical contradiction, nothing useful can ever come out of such a discussion.