Are there any sects or significant Rabbanim, other than those in Chabad, who hold that the late Lubavitcher Rebbe (now that he has passed away) is mashiach?
Yes, Attached is a Psak Din that was signed by hundreds of Rabbonim (many non-Chabad Rabbi's) That the Rebbe is Moshiach. Note that the date of this Psak is 1998-2006.
If there are those that do believe him to be the mashiach, do they think that he is the only option or that others (whether we know who they are or not) can also be candidates?
No. The answer to
who Moshiach is? is completely a Halachic issue, and not based by elections or the popular vote, but rather qualifications that the Rambam (whose Yortzeit is today) wrote, which are so unique that only one in a generation can meet them.
I hope that I answered your question.