1. How high off the ground should I place the nest boxes?
With one exception, wherever it is most convenient to you. Chickens usually don't care. There can be exceptions, but usually they are happy with high or low, mainly what they get used to.
Some people put them on the ground.
Some raise them off the ground a bit, say 12 to 24 ". This does tend to keep the nest boxes cleaner. If you leave about a foot or more clear under them after the litter is in, the area taken by the nesting boxes does not take away from available coop space. Your coop is big enough for eleven chickens but your run is fairly small. The extra space may be important to you. I got mine a little too close to the ground and they tend to fill the space under the nesting boxes with wood shavings. They are always scratching and they kick them under there but do not go under there to clean them back out.
Some people raise them to a height that they do not have to bend over to gather the eggs. If you put them up four or five feet, you might want to add a perch or something half-way up to help them get to the nests. If they have a good landing pad at the nests, they can fly up four feet, but a midway perch seems to help.
2. How high should I place the roost.
Clearly higher than the nests or anything else you do not want them to sleep in or on but as low as possible. It is not an everyday thing, but chickens can hurt themselves getting off the roosts. The larger breeds are more at risk, but any of them can hurt themselves. When they jump down, if they land wrong, they can injure their legs. In addition, they are clumsy fliers, even the small, light, flighty breeds. When they jump down from a roost of any height, they spread their wings and try to fly down. The higher the roost, the more room they need to fly down. They can bang into nesting boxes, feeders, waterers, walls, other chickens on the ground, anything that might be in their way. That's why I suggest as low as possible for the roosts.
3. Should the roost be higher than the nest boxes or vice versa or can they be equal height?
This is the exception in #1 above. Even if they cannot roost on top of your nesting buckets, they can roost in your nests. They poop a lot when they sleep. The roosts need to be clearly higher than anywhere else. If they are pretty close together, 6 inches may be enough difference. Clear across the coop, it may take a foot of difference. In your case, I'd make the roost higher than the top of the buckets to help keep them from getting confused.
I assume you have a lip on the front of those buckets so they cannot kick the bedding out.
Good luck!!