Can a single threat, used to cancel a maneuver, be used to end a spell that has had the concentration maneuver used to sustain it? For example, if a PC has a barrier spell up can a single threat on that PC be used to end the barrier he/she cast 3 rounds prior but was sustaining via concentration maneuver?

Yes. It's a manoeuvre and one can indeed be used to cancel a manoeuvre. Thus, can cancel the concentration manoeuvre and the barrier (or other spell) fizzles and dies.

yes and no. In short have a discussion as a group on how to handle it.

Does the spell instantly fail? Or, Does the caster have the opportunity to spend another maneuver to maintain the concentration?

Both could work, and this might actually be a really good question for the Devs via the FAQ to see their intent and help groups form their own intent from that. As with all things genesys the specific context can really matter, but consistency within how the group treats it internally is the most important factor, at least to me.

My players were enthusiastic about using this to thwart NPC spellcasters, but they cried foul when I did it to them. So right now this particular use of Threat is under review. My inclination is to not cause concentration to automatically fail when Threat is spent, but instead that the target must make a Discipline roll with a difficulty equal to the amount of Threat spent.

there is a new answer from dev's in the faq about this