Alternatively, what do people make of Louis Joseph, being born in 1778 as a twin with his sister Marie Therese?
Depends, what does this Louis have as a personality? If he's like his dad, France is screwed. If he's like Louis XV France is screwed. And if he's like Louis XIV, you guessed it, France is screwed.
He'd be 11 when the Bastille falls, 15 when the monarchy gets abolished. Old enough to have ideas of his own by that point, so they can't farm him out to the Simons in the hopes he absorbs revolutionary fervor (unless he's a dimwit). Chances are he either goes to the guillotine alongside mom, dad and Tante Babette; gets quietly done away with in his cell but nothing is ever proved (something like LXVII OTL), or he gets away and becomes Charles X come early. Madame Royal was a rather staunch supporter of the Ancien Régime during the Restauration, so if his personality is anything like hers, wait for it, France is screwed.
As to Louis XVIII having kids, best way for it to happen would be for him to lose weight. He had diabetes because of his obesity which made him waddle rather than walk (part of the reason he didn't go through with a sacré for himself, even though he apparently wanted to). The obesity would've also made it that he couldn't make his soldier stand at attention. So the best chance forhim to have an heir would be Madame carrying one of her miscarriages to term. And even then, it's debatable as to how much of a middle-of-the-road character he'd be. Angoulême was Anglophilic, Orléans was out an out liberal, Berri and Madame Royal were both in favour of Ancien-Regime influenced policies (and they were all born in the 1772-1778 bracket, and lived through almost all the same things from 1798 on), so I'd say a son for the Cte de Provence or a twin brother to MR would stand a fifty-fifty chance of ending up like ANY of them.