It's a classic problem with trailer frames that have rails too close to the tires (often by necessity) for proper placement and orientation of the dampers. AT has the same problem with their ITS as do the couple of OZ trailers that I've looked at.
For springs I would look at using coil-over springs instead. Can buy them by small increments of a huge range of known spring rates and if you miss and do not cosmetically damage them most vendors will exchange them for a different rate.
About the only thing that bothers me is the tongue. A common on these various off road forums construction method is as you have it, but there will be a very large Moment at the front edge of the front cross-member. You may or may not get away with it, but a heavier trailer built like that will fail the tongue tube at the front cross-member. Tongue tubes not only need to be continuous, they also need to be under the rest of the frame. Better is if there are two tongue tubes in an 'A' shape where the rears of the tubes tie into the side rails where the front spring or suspension anchor points are. I'm guessing that you didn't go down that trail based on being able to jack-knife the trailer into a much sharper turn.
Since trailers do not articulate (no force available to do that) I wouldn't worry too much about the loss of travel that a sway bar brings with it. However since occupant comfort isn't a priority with a trailer, the springs rates don't have to be any softer than one that doesn't break the eggs, foam the beer, or fatigue the structure. Getting into that rate range may negate the need for one.
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