Great thread.
I am going to comment on the agility (lol its what I know best). When clickering training or having a reward marker, it is done at a HIGH rate of reinforcement at the beginning of training the 'new' behaviour, the same as any other reward marker training. Then moving to a duration and a THEN a variable reward marker and reinforcement.
Agree with others, reward/reinforcement can be any number of things, food, tug, ball, toy or just being released to do the next obstacle, especially for a dog that 'gets off' on doing agility (although that isn't the best lol).
Absolutely the dogs have to be trained to wait for their reward/reinforcer once they have been built up slowly to that level, that is why we often pair the clicker with Yes!. Not to mention 'jackpots' for an amazing performance of an number of different behaviours that have been trained. One doesn't miss an opportunity to mark and reward a spectacular performance in training (i.e tight turn, nailing a nasty weave entry, contact performance or even a stellar startline stay) , to do so would be foolish and leaving it up to chance for when it counts. And of course thats also variable.