I don't use Snelgrove boards so I'm not the best one to answer about those.
In general young bees are better than acceptance than older ones.
Yes, I tend to keep the odd spare queen in a nuc as a 'just in case'
For the useless colony; I have caged an unwanted queen for a week before removing her. In this way you have no queencells in the hive (usually!)) and no eggs/larvae from which to make a queen so the bees have no choice to accept her if they know what's good for them. [This isn't something that I have seen written in books though. Maybe it's just my fantasy that means I think it works!]
(I have seen it, where a colony accepts a new queen after being queenless for a few days, however they are in the mood to generate queencells at this stage, so they continue to do so as they have material to work from. And the new queen swarms out a few days later).
May your bees read the same books as you do.