Couple observations about the scenario posited:
- Anna de Medici wasn't popular at Vienna (IIRC it had to do with her Medici blood, and Cosimo III's marriage to a French princess didn't help matters), and said unpopularity transferred to her daughter, the Empress. So, if Sigmund Franz lives then Anna's perhaps still less popular, but since she's not in the same position as OTL, it doesn't matter. Leopold would only be interested in Klaudia as a wife if he's a widower and she's the last of her line. Remove one of those factors (like Empress Margarita Teresa living longer or Sigmund Franz having a son) and the match is considerably less likely to happen.
- Maria Antonia and a hypothetical son would be more distantly related than her and Maximilian II. OTL, through her dad, they were second cousins (he's a cousin in varying degrees through his mother (a Savoyard princess descending from Felipe II), her mother (a Spanish infanta born of a match between his cousin once removed (Maria Anna) and her uncle (Felipe IV)).
By contrast, Sigmund Franz's son is born of a princess of Pfalz-Sulzbach (who's mum is a Nassau-Siegen), and Sigmund's mom is a Medici (daughter of a Lorrainer princess). His closest relation to Leopold I is through Archduke Karl of Inner Austria (also dad to Emperor Ferdinand II, Queen Margarethe of Spain (Felipe IV, Anne of Austria and Empress Maria's mother), and Maria Magdalene, Grand Duchess of Tuscany (Anna de' Medici's mom)). Suitably foreign I'd imagine.
-As to Saxe-Lauenburg surviving, I'm not so sure. The dukes there have got a bloodline so concentrated its making Carlos II and the Ptolemies cry (with joy or fear is anyone's guess). However, Julius Franz is in a reasonably good place since from what I can make out from the Askanier family tree, all his female cousins/kinswomen are past childbearing age or already married by his wedding in 1668, save one: Eleonore Charlotte (b.1646), daughter of Franz Heinrich of Lauenburg and Juliane of Nassau-Siegen (sister to his OTL wife's Siegen mother). So if Julius can avoid marrying Eleonore (maybe she's already wed by '68 - though she only wed in '76, IDK why), he MIGHT be able to marry to counteract the Lauenburg inbreeding. Don't know who he'd marry though, since Lauenburg wasn't overly important by then FWIG.