Okay unfortunately I cant help you with your questions except for one :)
And that is how you can extend his trot.. Yes, not very helpful, nothing to do with eventing, but its something I hope
Anyways, a way I know, which I also use on my horse, is when you trot, sit up and down for longer, especially up. Try to get just a little behind the rhythm perhaps, however dont let him slow down..! (This method is also used to slow a horse down, but this is not what we want this time). Keep your pace, but continue to rise slower, and stay up just a little longer. It might feel a little awkward, and be hard to start with, but you just have to get out of the rhythm until he falls back into your
rhythm and not his own - your rhythm being slower, forcing him to take longer strides.
This might not work on all horses, but if you do it right (make sure he doesnt start slowing down), and you keep doing it constantly, it will work :) Thats how I get my horse to do a nice, smooth and long trot.