All of the heretics are illegitimate.  And the legitimate heir to the throne is in Australia.

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Are they illegitimate because they're heretics, or because they descend from Elizabeth?
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You understand that Elizabeth had no descendants, yes? So unless you're trying to say that heretics cannot hold a secular position of government (even more absurd), I'm really not following the logic. 

Pope St. Pius V, Regnans in Excelsis:
St. Pius V excommunicated those who would consider her to be the Queen.  Therefore the line of British "royalty" is copmletely illegitimate.  Given the reasons for his commands related to Elizabeth, one would gather that St. Pius V would also disapprove of any who would pretend that the successors of Elizabeth were legitimate also.  If she ceased to be queen, then here successors are not legitimate royalty either.  St. Pius V excommunicated anyone who would treat her as having any authority (i.e. by obeying her orders).

Ironically, Elizabeth was no queen even before this, but had usurped the line, which now resides in the son of a truck driver from Australia.

Sorry, are you really saying that a law excommunicating and deposing someone who has been dead for four hundred years is attracting some kind of late sententiae re-administration on totally different people who kind of meet the same description? I assure you, that isn't how law works. 
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I really don't care about the trucker; the English throne at any given time in history has had oodles of legitimate claimants vying for its title; if all it takes to completely delegitimize a reigning king is a single competing claim, then England has never had a legitimate king.