Thanks for taking the time to make a detailed report. You're right, we had not tested private window interactions enough with this latest release. We've just fixed this in the latest 1.51 release.

We have long wondered whether we'd be over-complicating things to allow tabs to be restored (either individually or as a group) into a private browsing window. So far we have decided not to over-complicate the user interface with private window options, but please do let us know the scenarios in which you would find it useful to restore tabs to a private window, so that we can re-think the trade-off between UI complications and this functionality.