Any estimated or guesses on when the patch and season will hit ?
Also besides the new sets , the skill buffs and changed legendaries are available immediately outside a season character right? Like corpse explosion and gauntlet damage buffs?
Speaking of which does the extracted power get updated in Kanai or do you need to find a new version of the legendary and extract it again ?
First of all, the season 20 will end. Most likely on a sunday somewhen after the PTR ended, most likely within 1-2 weeks.
Then a week or two downtime and season 21 starts. Its really hard to say it more properly since it really depends on how much is to fix in the real patch.
The new patch will likely hit on a tuesday/wednesday in between the seasondowntime. From then on you have your corpse explosion and the items 
My best guess is the following, based on recent seasons: PTR end on 11 June. S20 end on 21 June. Patch on 30 June and S21 start on 3 July. But only time will tell, of course…
Seasons always starts on friday
They still haven’t given the 2-weeks notice for season end, and there’s usally at least a week before the next season starts, so you probably still have at least 2 and a half weeks to wait for the patch to hit live.