Your initial thought was correct. All runs were earned because the standard rule is that you cannot assume the DP, as you say.
Occasionally a scorer might make a "pitcher friendly" assumption, though. For example, suppose runners on 1st and 2nd, easy roller to 3B. All he has to do is pick it up, step on 3d and make a routine throw to 1st. If he boots it or lets it go through his legs, fair to assume a DP there. Or if he steps for the out but then throws it away, again fair to presume the DP if a routine and good throw would've easily gotten the runner at 1st.
You might get another exception in an odd situation here and there. Suppose in your sitution, your SS's throw was wide and pulled 2b off the bag, but 2b successfully relays for the out at 1st. You might charge an E there and for the sake of calculating earned runs, you could presume the DP.
But in the more typical situation you present, where the relay throw is the problem, no E, no DP presumed, all runs are earned.
