> the fact that they are ultra-conserved and seem to convey no survival advantage is a severe blow to RMNS
No, it isn't. It's quite simple: those genes that provide a benefit to the animal tend to be reproduced at a higher-than-normal rate. Those genes that are detrimental to the animal tend to be reproduced at a lower-than-normal rate. And those genes which simply do nothing at all are under no pressure to be deleted. Genes which are "off" will be left in place until such time as random replication errors cause changes, which may well be a very long time indeed.
ANALOGY ALERT: It's like computer programming, but without the programmer... as the program is updated, those sections of the code that are no longer needed might be tagged as "comments" and left in place.... they do nothing but sit there and take up a tiny amount of space. As several thousand generations of iterations occur, quite a lot of such "comments" can be built up; they do nothing, either good nor bad. If at some point the comments are deleted... the program runs just the same as it did when the comments were in place.
Well first, they are not genes, since they do not code for proteins. Secondly, although there may be no pressure to be deleted, there should certainly be no pressure to correct any mutations occurring on them. They are pristine. No changes. And the analogy is "good" in that programs don't mutate on their own. Darwinian evolution requires mutation.