Seasons always begin on a Friday at 5PM server time (CET in EU, PST in NA) and you can prepare the run in a way that you have the rift guardian spawned and kill it immediately when the season begins. However, don't complete it(don't kill the Rift Guardian) before the Season Start notification has popped up. You don't actually need to have a seasonal character to collect the rewards from a Challenge Rift there later. Items from season at end that where in stash will also be mailed yeah.You can start the Challenge Rift before the season begins, which will give you a Challenge Rift Reward Cache containing a good bunch of crafting materials, 475 Blood Shard s and 5.1 millions Gold to kickstart your leveling process. Kenais cube powers at the end indeed get merged(note that this season will be special so once it becomes non seasonal you have to reactivate any active cube powers you had.) The paragons on non seasonal stay untouched, at least till the end of the season where they combine so for example 800 non seasonal combined with 450 you got in season ends up at 890(note this example is not full accurate if you want to have a accurate example there is a site that shows you how much you will end up with can't post it easily now since on mobile ) (mail icon on non seasonal is in left corner.) your paragon in season will be 0 unless of course you already played a other char that season in seasonal mode that hitted 70 and got paragons in it. They do merge yeah, it works like this with rebirth, you rebirth your character all items you had on it will be in a mail that stays for 30 days in your non seasonal mail. Yes I recommend rebirthing, if you then have one of every class you have enough slots free to also make hardcore versions of each class if you like.

What does rebirth in diablo 3 do