Regarding the Carlos-Eulalia marriage, the couple were very good friend and remained so till his violent death in 1908 (one of several royals Eulalia knew who were killed or died in violent deaths -Nicholas II, George I of Greece, Carlos I of Portugal, the Duchess d'Alençon, Elisabeth of Austria...). Eulalia wasn't too keen on the marriage, though she didn't dislike Carlos himself, but she complained that she was not fit to be an infanta, let alone a queen (at least it'swhat she says in her memoirs). Alfonso XII, urged by some ministers who were against the Luso-Spanish alliance, decided to intervene and no marriage took place. As you know he evtually married Eulalia's niece (or second cousin once-removed, depending on which family link you follow) the same year Eulalia herself got married to Antoine d'Orléans.
As for Montpensier, he was certainly a favourite for the infanta Eulalia. When Alfonso XII died his son was the only male descendant of Fernando VII of Spain IN Spain, and both men became the centre of attention at funerals and family gatherings. When he died in early 1890 his death shocked Eulalia, who was near her last stages ofpregnancy. A month later she gave birth to a baby girl that died during the ordeal -asfyxia- and Eulalia's relations with her husband would forever be destroyed.
