If you make wider feet at the front it should wobble too much, and without digging it in you can still move it. Might want a little more tranny at the end of the ramp though.
Drill holes in your feet on each side, drive 2'-0" of all thread into the ground with a sledge and washer and nut the top. 1/2" all thread is cheap and will drive into the ground fairly easy, then you don't need a trench and you'll have good stability.
Take the drop in and move it over to that treehouse platform you already have, destroy that (probably) wobbly-ass platform you made and use the wood from that to build a transition/jump to get on the rail. Also you could probably get away with removing half the 2x4s you used on the drop in
