Yeah that's pretty much exactly what happened to me except I was in a church.
You can still fly it in ATTI mode but it needs a lot of practice as you have to manually correct for drift, and you can't practice because as said there is no way to manually put it into ATTI mode
Not sure, the problem is when it switches to atti the controls respond completely differently, so it feels like you are not in control but in fact you are, it just does different things to what you are used to.
If you had no response from the drone at all, that must have been something else. Not sure what it would do if it lost GPS and also lost the remote signal - probably just attempt to hover in place, since it can't return home?
i flew indoors once, with no dramas, I was a total noob and just got lucky? also didn't do anything tricky, just tiny movements. does tripod mode help? or does it make the responses too slow?