Regarding the lost root on D41510e version, I realized that the root might not be completely lost. First of all, t-mobile's own "my account" app still reports the phone as rooted. Also, I can see /system/xbin/su (and other relevant files) as well as /system/bin/.ext/.su. However neither in the terminal nor in adb shell can I successfully become root by running any of the su binaries I see. Next I will try to run root checker and see what it reports, but I am hoping that the presense of su binaries might allow root recovery somehow and that someone might have enough knowledge to suggest a course of action.
(Btw, I was running an adblock app and the modified hosts file is still on the system. So I suspect that if/when a large update comes in the future, the update will refuse to run because of these modifications.)