TeXstudio uses Hunspell as spelling engine. Replacement proposals come from there.
It appears, that Hunspell does not consider Númenor a replacement for Numenor. In contrast, Númeno will have a suggestion for Númenor. This is beyond the control of TeXstudio.
@Qrrbrbirlbel: Your information concerning "Add to dictionary" behaving like "Ignore this word for spellchecking" is somewhat outdated. This was the case in older versions of TXS. By now, user words are added to the Hunspell-Dictionary at runtime using the Hunspell_add() method (and still stored in a .ign file on disk). As far as I know, this is equivalent to having the words in the dictionary in the first place.
Back to a solution:
As pointed out in the comments, you really don't want to replace every occurence manually using the context menu. A macro is the way to go. For just a few words, one macro per word is fine. If you have more, you could write a generic script, that checks the last word against a list of possible replacements. This script for autocompletion may serve as a starting point. It does not exactly do what you want, but the logic is quite similar and should be easily adaptable to your problem.