Don't attempt to copy my lower front lines then that are spliced the entire first 2m from the bar to combat line wear at the cross point of the back lines
Seems like I am talking to a wall about the unimportance of the strength of sewing, even a drop super glue is sufficient to lock a splice in place. It's only there to lock the splice in place when you handle a line without any tension and before it's connected to a pigtail, leader line etc. It's per...
As I said before: stitches are not load bearing with a proper splice. So you don't need strong thick thread, no more than 10 stitches and no back and forth, just melt the loose ends to stop the stitches become undone.
And yes, you need something on a sewing machine to guide the line.
With a long enough bury (100x diameter or more as you did) the stitches are only there to keep the splice in place when handling the line. Under tension it will stay in place. Long bury, taper and only a handful of stitches is the best splice you can possibly make.
Why use DC401 for bridles? That line will shrink over time. Normally for bridles sheeted lines are used, they don't shrink as much. More and more brands switch to unsheated lines as they are thinner to save weight and drag. But yes, that comes with other problems like wear and shrinking. Should be ...
Impossible to land a megaloop let alone double/triple loop without some kind of loop upwind of the rider to kill the downwind momentum. If you don't do that loop to slow down you will be traveling at almost the same speed as the wind and just drop out of the sky like a brick. Even with normal jumps ...