Im jealous, that trail is so pretty!
If your tying her to where shes getting her feet tangled, you are giving her far to much rope( what a gem for standing there waiting for you to help her, i had an older arabian X mare just like that). You want them to be able to put their polls at wither level for comfort if you are tying for a long period of time. but no further. I think all horses are like that when you ty them over good grass. lol they WANT it! until that is they learn they cant have it and no amount of pawing is going to get them that grass!
I discovered my mare pepper had that backing out issue (same thing, terrified of the step down) when i bought my two horse straight load - Until the first time trying to back her out of it, it never occured to me that in larger trailers i ALWAYS turned her around and walked her out.
What I did to cure this backing out problem was pressure and release. first of all i chose a day that i could devote sometime to this with out any time constraints( ended up taking me about three hours from start to finsh. I started small, and would just let her take a step back (you have to release at the very moment they even throw their weight back.) and pat her and talk to her, repeat asking for a little more each time. I just kept at the pressure and release until she would step down (i would also say step when it was time for the step down). at first she would freak and jump right back in the trailer ( at that point i was back a square one and had to start over at the beginning). after a few go arounds of this jumping back into the traile (took less time inbetween each go around to get her back to the step down point), i did finally have to just throw my weight into the lead rope to stop her from jumping back in. SHe still had her front feet in the trailer at that point but me making her not able to jump back in gave her a moment to think about what she just accomplished and the fact that i had NOT just asked her to back off a cliff:)... along with alot of petting and good girls. it was easy peasy from there, you just have to prove to them that they are capable of that step down. Now that i have a bigger trailer that i can turn and walk out of, i still make my horses back out of them to keep up on the backing out thing.