I am not a carpenter and I have never built anything in woodshop more complicated than a soda-can lamp or a small 2 ft ramp for skateboard/rollerblades.... though I have helped build foam block spring floors for gymnasts
Having said that, I think that your drop in will break if it gets used regularly or has more than 2 people standing on it/using it.
Your braces (especially that bottom one) should have diagonal cross beams on the left/right and front/back to keep it from shifting to 'parallelogram' shape (sorry, don't know the term). You can probably get away without using one on the top frame, but I would put more beams under the platform that people will be standing on, and I would double layer the wood serving as the floor (making sure that seams do not align).
It might be a bit of overkill, but its probably better to over-engineer it the first time than break it mid-season and try to fix it in 30 degree weather.
Your 'ladder' supporting the ramp of the drop should be almost as wide as the ramp itself. No more than 4 inches of overhang on either side.
If you do not put a support in the middle of your 2-piece ramp (either wood, a dirt mound, boulder, etc), then it will move and eventually break. If it were me, I would build a four leg support at the top of the lower ramp section