This translation also reminds me that I am in search for an elegant way of expressing "must be/surely is". Tankave looks like a cop-out. Or maybe not.

Perhaps it is significant that in several languages (I know) this is expressed with a dative/allative: for sure (E)/na pewno (PL)/bizonyára(H)

+Tamas Ferencz I'm not very familiar with dative/allative outside of Q, but as an alternative to tankave (which may be fine), one could derive from tanka (which seems well-attested) an abstract noun like *tankie "firmness" and either use it as directly as an adverb or inflect it for instrumental: *tankiénen "with firmness". It seems like Tolkien used this idea a few times.

+Tamas Ferencz I wouldn't call tancave a cop-out. In cases like this, where no actual compulsion is involved, it's definitely preferable to mauya .

Another tricky point is 'single man' (= 'bachelor'). Would erya nér or *eryon do, or would it rather suggest 'recluse'?

For 'in want of a wife' I think I'd try mangesse (or *mangea) verio.

+Björn Fromén I didn't know mange at all, but if we go for " lacking" we could say penya vesseo

As for "single" we could approach it from "unmarried". *lavesta?

I've just realized I am now veering into Captain Obvious territory. "An unmarried man is lacking a wife." Duh.

Thinking about "single" I am also thinking that the three adjectives could diverge a bit and get fixed in their meanings: erya could be the unqualified "single" (in line with tatya etc.), erinqua could be "sole, unique_", and eressea "lonely".

+Tamas Ferencz Not more seems to be known of mange than the QL gloss “lack, want, shortage”. But that sounds more pregnant to me than the mere pen- ("without, not having").

+Björn Fromén you say that mauya- involves compulison, but from the same root we also have maure "need" (http://eldamo.org/content/words/word-845601899.html) so I think that compulsion can also come from the need itself, not necessarily an outside force. Just like or- is also a form of inner compulsion.
Nevertheless, mange is indeed not to be overlooked.
eldamo.org - Eldamo : Quenya : maurë

+Tamas Ferencz I agree that mauya- can probably refer to inner needs as well as outer compulsion. But when 'must' (as here) means something like 'is certain to' or 'can impossibly not', I still think tancave is a better solution.