[Legolas]

For example, I may be wrong on this, but I had always understood that doubt remained over the fate of Tuor, notwithstanding that it was sung that was "numbered among the eldar race".

Immortality and Mortality being the special gifts of God to the Eruhini (in whose conception and creation the Valar had no part at all) it must be assumed that no alteration of their fundamental kind could be effected by the Valar even in one case: the cases of Lúthien (and Túor) and the position of their descendants was a direct act of God.

Of this Order the number is unknown; but of those that came to the North of Middle-earth, where there was most hope ... the chiefs were five.

i.e. 'We don't know how many Ainur there are, but of those that were sent to our area of Middle-earth, we know of at least five.' In that case, those five being chief is simply a guess because of their being known (while any others remain a mystery) and Saruman referring to the staffs of the five wizards. The Istari essay was written and originally intended to be a part of an index for Lord of the Rings. I'm not sure if the planned index was to be a separate, 'outside of the story' inclusion in publication, but I think Tolkien would've not done so - meaning the essay would've been a part of the recorded history just as the appendices were. It's very possible whoever writing that essay inside of the story would not have known everything there is to know - it's said that only Cirdan, Elrond, and Galadriel would've known of the Istari's origins (during the Third Age); naturally anyone else would be uncertain of the number, knowing only the five that have 'been said' to come at the bidding of the Valar. Glorfindel too came from Aman (as noted in HoME XII) - any observer (not being omnipotent) might wonder if there were others (whether they be elves or wizards), and if not wonder, at least not reject the possibility of others being sent.