3 of them are missing translations, but are finally on track to be released. Sword of Destiny is a short story collection coming out this year, which was skipped over in translating before. Chronologically it's between The Last Wish and Blood of Elves. Then Tower of the Swallow comes out next year, and Lady of the Lake the year after that.
As mentioned before,
all the missing books were translated by the community in the Witcher forums.
Both paths come together in Act 3. It's 2 sides of the same story more or less. Each side has information about the main plots that the other side doesn't have, so you don't have the absolute full picture without seeing both sides. But the events of both sides still probably happen more or less even if you weren't there.