Oh Laura's real alright, wasn't it stated in the game that she was in the hospital at the same time as Mary? Maybe she came back to Silent Hill, to reminisce or something, but found nothing except for James and Eddie.

The thing you have to remember is this, it's that the Silent Hill that James, and Eddie and so on enter is totally different from the REAL Silent Hill, they're in another dimension totally. Light Silent Hill is not the REAL Silent Hill, Samael in SH1 sorted that out.

For all we know, Laura could be walking around a Silent Hill that's totally normal and populated by people, so it's not so weird that she's involved in what happens.

Notice she never appears in Dark Silent Hill?