Before the purists step in and scream "of course there's a possibility that the princes' lives will be different and they would survive"...
Lessee...1666..The fire is worse and reaches Whitehall...
Long minority and regency, no matter where we go on this...
List of Monarchs (assuming everybody dies on schedule)
Charles II: 1660-September 4, 1666
James II: September 4, 1666-September 5, 1666 (Oh lets be nice and say he dies a day later of complications.)
James III: September 5, 1666 - June 20, 1667
Mary II: June 20, 1667- (with regency going to about 1680 or so, although maybe joint rulership is invested in William earlier)
I'd say you get the marriage at about the same time as OTL, given Mary's age.
I don't think the war would end any faster but you might get a better set of treaties without Charles II's bad-faith negotiations. The Treaty of Breda was signed at about this time in OTL, so it might be done a bit earlier. I doubt the Franco-Dutch alliance would hold much longer than OTL but I can't see the third Anglo-Dutch war coming off in this TL, without Charles II signing the Treaty of Dover. Would Loius XIV still invade if there was an Anglo-Dutch alliance from about 1668 or so?
The marriage may be proposed at that time or maybe a bit later (around the time the *Triple Alliance was negotiated)...maybe by anti-Orangists to get him out of the Netherlands altogether. Although if Louis does invade, then I can see the trajectory of events making William the Stadholder ITTL by the time the marriage is celebrated.
William is still third in line so the dynastic reasons for the marriage still stand, and they might come to the fore a bit earlier. The stuarts have taken som pretty had dynastic kncoks her, so the drive for heirs would be pretty strong. In OTL, it was Charles II who was against the marriage to William III, while Parliament favoured it over a French match, for all of the obvious reasons.
Just some thoughts...
David