3L and さゆり are entirely different persons, the only other name ever used by Sayuri that I am aware of is さゆり姫, and that seems to be limited to the profile on Silver Forest's homepage, at least no CDs seem to have that name listed in the booklet. The twitter posted also doesn't say anything about illnesses, which was weird and got my hopes up that these news were maybe wrong/misunderstood/whatever, but well, that explains that.
Was surprised that there were only Best Of CDs planned for Reitaisai, but this is probably why. At a loss for words here, but I really hope that she gets better, I really do, I've always loved Silver Forest's songs, particularly hers.