First of all the room looks great!! It looks like you thought of everything! You have accomplished a lot in a short time.
I like the idea of parking that train, but as you mentioned that is a short, steep run! Why don't you set something up temporarily like I did to see if my engines could pull the length train I envisioned using up the grade. I happened to have my Woodland Scenics foam grade riser sections from 25 years ago and tested 6%, more than I needed. If we can meet up at the Greenberg show on Saturday, I will bring them along and you can keep them until I see you again.
i have built open grid and L-girder. Both worked fine for me. I say build what you are familiar with, unless someone else can convince both of us to the other.