In countries with high latitudes, are the times when prayer is forbidden estimated in the same way as the times for obligatory prayers, or is it sufficient to estimate the times for the five obligatory prayers, leaving the rest of the time available for supererogatory prayers?

There is no prohibition against praying in regions where the signs of the prayer prohibitions are not apparent. If the sun does not actually rise or set in these lands and similar places, then there is no time during which prayer is forbidden. Rather, they may perform supererogatory prayers at any hour they wish, whether by night or day. Analogous to this is the eclipse prayer, which is only performed when the eclipse is observed.