Mary was forced to marry Lascelles because in the aftermath of WWI most of the suitable Protestant Princes were too German, therefore a suitable match to a very wealthy British Lord was considered as the only suitable alternative.
Mary
did marry her OTL husband, but he died in an accident a few years in, much like Hirohito's wife.
There was also the Japanese reaction to a foreigner marrying into the Imperial family and how this would have been accepted.
I did note within my post that the Japanese genrō and aristocracy are scandalized by the Emperor's decision--but the Emperor is now using a vested interest of drawing public support for him and his new wife to keep them from being too
finicky about it.
Of course, the situation in Europe and Asia with the resurgent Mongol Empire and Communists across the Channel might have them consider the need of scelling an alliance as so.
This is my justification for saying 'hey, this may kinda work'--the British are scared of a violent communist state across the Channel, Japan is afraid of the racist xenophobic ultranationalist Mongolia across the South China Sea. So, a marriage would perhaps
pan out in some sense; despite the sentiments against it by many in both courts.
Japan and Britain's more conservative elite permit it entirely on the premise of containing communism and creating a deeper alliance against them and the Mongol state.
A much more plausible marriage would have been to a British actress or socialite, I think even a daughter of a titled family would have been scandalous.
TBH I had no idea who else to pick--I was considering having Hirohito just marry a commoner in Japan (much like his son did IOTL), any suggestions perhaps I may replace her with?