Ok so let's say Adeliza conceives on the wedding night, getting the tail end of he husband's fertile years, and has little Richard Adelin is born around mid to late October. Thus, he's barely 14 when he ascends to the throne, but he probably isn't put to a Regency, but "advised". Unless he looks particularly young, he's probably going to be seen as a medieval man, and thus capable of rule. His mother probably doesn't remarry immediately, however, and may marry within the court to stay with her son.
Now Richard Adelin doesn't necessarily have to be an only child, with the pressures of the hope of a dynasty on his shoulders. If Adeliza and Henry are match enough for one child, they can have two or three quite easily. With a brother of two, Richard goes from blessed only child to part of a family unit, and thus probably escapes the worst of the pressure and doesn't hate his father so much. However, if he doesn't have any brothers or siblings at all through his mother, then he is going to suffer from the weight of expectations. How that weight holds will depend on his personality and the support system of his family, in particular his mother. Matilda probably remarries ITL along the same lines as OTL, or perhaps closer to the throne, depending on when it happens. Thus, she might act as a support too.
Now I don't think Eleanor of Aquitaine will work. She's probably still going to marry to the King of France ITL, and by the time that goes Richard is going to have been married off, particularly without male siblings. But, I could see a match between Richard I of England and Constance Capet, Princess of France. Other options include a daughter of Stephan of Blois, Constance of Antioch, Margaret of Navarre, Matilda of Savoy, Eudoxia of Kiev, Richeza of Poland and Berengaria of Barcelona. I personally think that Constance of France is the most likely option, although Constance of Antioch might be interesting.