Mary Queen of Scots Marries Robert Dudley

I'm assuming you mean the 4th Duke of Norfolk. It wouldn't have been possible as that time he was already married to Margaret Audley. She wouldn't die until 1564.
 
The only way I could see Mary marrying anyone Elizabeth put forward was to obtain the benefits of doing so. And as discussed earlier, Elizabeth as a ruler was reluctant to provide those benefits. So while its a possibility, unless it came along with a hefty benefits package, I could not see Mary marrying him.
 
Norfolk did scheme to make it happen OTL but Elizabeth caught wind of it and had him imprisoned. Elizabethan intelligence services where actually quite effective...
 
Elizabeth becoming fatally ill earlier than she died otl might be a option?
Possible but not likely. Elizabeth had done an excellent job at making the Protestants hegemonic economically, politicaly and militarily very quickly at the beguning of her reign. They will form a rather formidable opposition to Marry. A great political mind would probably be needed to handle them and Mary Stuard didn't have that, in fact she wasn't able to handle a rather similar situation in Scotland.

(Admitedly I might be a bit biased since unlike other posters on the thread I quite like Elizabeth I).
 
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