All the stuff you commented on is what the past 55 or so pages have been about, "When does surprise end?" because the assassinate ability needs to know this, but it is never directly stated in the rules as surprise ends when X happens. I have stated my opinion on the matter of when surprise ends, and that matches with the way Mike Mearls handles it too, surprise ends at the end of the surprised characters turn when they can take reactions. If this isn't your take on those rules after 55+ pages of debate whatever else I say isn't going to change your mind.
As to this part here.
The monk does know where the attack came from, because of the rules for unseen attackers and targets on page 194-195 of the PHB. The relevant part is.
If you are hidden-both unseen and unheard-when you make an attack, you give away your location when the attack hits or misses.
So normally the monk being blind would have to guess at the square his target is in, but the assassin has revealed his location by making an attack. The monk by using his reaction to act immediately after the attack hits is able to interrupt the assassin before they can move to another location or make a hide roll, so the monk knows where to attack with the deflected missile. Being blind the monk is still at disadvantage on attack.
The deflecting missiles part is all 100% by the rules, the only thing that isn't spelled out clearly in the rules for some people is when surprise ends. If you rule that surprise ends at some later time than after the monks turn is over the chances of them reducing the arrow damage down to 0 so they can throw it back is greatly reduced as they are dealing with a critical sneak attack. But even saying the monk is still surprised until say the end of the first round, they still have a reaction and can still deflect missiles.
As to this part here.
The monk doesn't gain supernatural senses just because he's got a high initiative roll! He still hasn't noticed the assassin, has no way to see an arrow fly toward him (because he already failed his Perception/Stealth contest), and so cannot target the arrow attack in any way.
The monk does know where the attack came from, because of the rules for unseen attackers and targets on page 194-195 of the PHB. The relevant part is.
If you are hidden-both unseen and unheard-when you make an attack, you give away your location when the attack hits or misses.
So normally the monk being blind would have to guess at the square his target is in, but the assassin has revealed his location by making an attack. The monk by using his reaction to act immediately after the attack hits is able to interrupt the assassin before they can move to another location or make a hide roll, so the monk knows where to attack with the deflected missile. Being blind the monk is still at disadvantage on attack.
The deflecting missiles part is all 100% by the rules, the only thing that isn't spelled out clearly in the rules for some people is when surprise ends. If you rule that surprise ends at some later time than after the monks turn is over the chances of them reducing the arrow damage down to 0 so they can throw it back is greatly reduced as they are dealing with a critical sneak attack. But even saying the monk is still surprised until say the end of the first round, they still have a reaction and can still deflect missiles.