It indeed is rumoured that the youngest son, Laurent, is not Albert his son, but Aldo Vastapane's if I'm not mistaken.
As to an affair with Salvatore Adamo. I believe he always denied it. I think he just simply made a song about her and that's it.
And yes; Albert had a long lasting relationship with Sybille de Selys Longchamps from which his daughter Delphine was born, who in my opinion has a very striking ressemblance to late queen Astrid.
If Albert doesn't want to publically acknowledge Delphine as his daughter that is one thing, but for him to actually deny
it when her resemblance to his mother Queen Astrid is so striking
makes him look like a complete idiot, imo.