I'd question putting two swithback sidings in, and having your only possible run-around start or end in the tunnel. Also, is that a siding IN the tunnel? How would that work?
If possible re-design your continuous loop so that you do not use the curved part of any switch for the 'mainline'. That is what the big roads do and do for a reason. You'll be happier with the results if your turnouts turnout to a siding, not keep you on the mainline. I think you have three of these on what seems to be your continuous running track.
I'd shorten the tunnel so there was not a switch inside, remove the siding in the tunnel, change the inner switchback siding to a non-switchback and change your switch orientation to eliminate the problem mentioned above. You'll still have the action you want but without the potential hassles.
I wouldn't go below 9 3/4" radius just for the appearance of the train as you run it.
Modeling the North Pennsylvania Railroad Bethlehem Branch in N Scale