I agree with Portia that a combined grade class works best for gifted kids when they are on the young end. I would create a spreadsheet like aquinas, and assess risks as mentioned by Ivy. Then I would follow my kids' inclination as you mentioned they love the teacher in the third scenario... I would enroll the kids in the new school being started by a teacher who the kids love and who seems to get gt kids.
Some may say that if the venture is not long-lived, at least there would be no regrets of the woulda-shoulda-coulda variety: at least they had some length of time with the teacher they love who seems to get gt kids. Because gifted kids may benefit from change in placement from time to time to assure a good fit, even if the new school does not remain commercially viable, it may not be a risk per se, but rather one great year which may become a benchmark by which all other educational experiences are measured.
