I do something similar, but I don't have cross-app relationships like that. I'm still fairly new myself but I might consider taking the outside table, making a snapshot of it, and dropping it in to your "sandbox app" and then recreating the relationship. Might be kind of a pain, but if you sever the outside connections it would be safer (in my opinion).
for myself, I DO copy my apps and then designate the copies as "SAndbox" and add the date and change the icon just so I don't try to do live work in the wrong app. I also carefully make sure the most users do not have access to the sandbox at all so that there is no confusion between which is the working environment and which is not.