As a follow up on Tim's answers, have I a comment to the question #3.
(3). Why do some yuccas in bloom not set seed ?
As Tim write, do the Yuccaplants need help from either a specific species of Tegitecula ssp. or from hand pollination by humans. Some species of Tegitecula only pollinate one species of Yucca, and other species can pollinate several species of Yucca. For details look here: http://tolweb.org/Prodoxidae
But this is not the only reason why some Yuccas don't set seeds. I did for a long time think, that some of my Yucca specimens were sterile, but that is not necessarily true. I have now learned (by trying), that on some specimens it is only possible to get seedpods/fruits on the upper most part of the flower stalk, or in the outer most of the side branches, on those plants which have side branches on the flowerstalk. The reason to this might be, that Yuccas has apical dominance on the flower stalk. Now I get seed pods forming on all my Yucca's and I don't waist time or rare pollen to flowers which most like will fall af unpollinated.
Tim, this might very well be the reason, why you don't get seed pods on your Yucca reverchonii x thompsoniana.
For Rob, if you want to learn how to pollinate Yuccas your self, take a look here: http://www.bennyskaktus.dk/Y_hybrids.htm#pollinate_by_hand
Best wishes and happy pollinating.
Yucca bloom and seeding 101 - questions from a novice - Rob North Tennessee July 4, 2013, 9:34 am
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