And yet, Alexander's uncle George was arranged to marry a Bonaparte (half royals by traditional standards) and Christopher married a divorced and widowed American commoner. Was it because both women brought enormous wealth to the family?
Absolutely, yes! In all fairness, though, Marie Bonaparte was a highly educated and sophisticated woman. Thus, money could/would not be the only reason a man would want to marry her.
It is true though that most Greek princes back then, particularly prince Andrew and prince Christopher, were literally penniless and living on the charity of relatives or at the expense of wealthy wives.
It seemed if these women were allowed to contract dynastic marriages, why wasn't Aspasia?
One may argue here that she was marrying a king or, at least, a king in a king's place.
More recently, Prince Michael's marriage to Marina Karella was morganatic (although recognized by Constantine II)
This marriage was dynastically irrelevant. Actually, the marriage was morganatic as you state, prince Michael had to resign his rights to the Throne and Madame Marina Karella remained Marina Karella. King Constantine approved of the marriage in a social, non-dynastic, sense. While the monarchy was in effect in Greece, Constantine had no constitutional right to elevate Mme Karella to princess except through introduction of new law. But even again, the Greek constitution of 1953 (the latest in effect before the fall of the monarchy)
was not allowing the granting of titles to Greek citizens. Technically, therefore, Mme Karella would have to shed her Greek citizenship, become a citizen of
somewhere, then become a prince's foreign bride and lastly become elevated to princess by law. But who would dare do all that and for a personage that was dynastically irrelevant, and, more importantly, at a time, that political changes in Greece were torrential.
..... but Pavlos's to Marie-Chantal wasn't?
This case is entirely different. After the divorce, so to speak, between Greece and king Constantine, the latter can do whatever he chooses. And, as long as he is not a citizen of Greece, he may even disregard the Constitution of Greece, but he may not offend the Hellenic Republic. So, it was entirely up to him to raise her to
Crown Princess Paul or even to a suo jure princess, that is,
Crown Princess Marie-Chantal without the
of Greece. Finally, if she had married Crown Prince Paul while the monarchy was still in effect, she could/would now be called
former Crown Princess Paul of Greece (assuming/hypothesizing that such a marriage would have been allowed by the government). As matters evolved, nonetheless, she is not/cannot be Crown Princess Paul of Greece or Crown Princess Marie-Chantal of Greece, but she is
Crown Princess Paul or
Crown Princess Marie-Chantal - whichever her father-in-law has chosen.
I don't know what the specifics are, but it seems that marriages to Greek women were considered non-dynastic but marriages to extremely wealthy foreigners were. Odd.
I hope my explanation regarding the case of Marina Karella answered also your last concern.
. These days the University is on a strike so I have plenty of time to work on in and by the way I am finding ineteresting stuff to post here. Among the others, I am using for my report a book about Greek royal which is quite antimonarchical but has a lot of first hand documents in it so it is quite useful. I can't remember its title now , but when the university opens again in about a week I ll post it here. 
