Does removing a creature from the battlefield during the blockers declaration phase affect the damage the player will take?
Here's a breakdown of the five stages in the relevant part of the turn:


Declare attackers step


(1) attackers are declared
(2) players get priority (to cast instants and activate abilities)

Declare blockers step


(3) blockers are declared
(4) players get priority

Combat damage step


(5) damage is assigned and dealt
...



For the most part, you only check what something does and if it's legal right when it happens. So damage (dealt in stage 5) is only dealt by creatures that are still in combat at stage 5, and is based on their power at the moment stage 5 happens. Blocking (done in stage 3) is legal if it is legal at the moment stage 3 happens. So to answer your questions:

1) yes, since the creature is not still in combat at stage 5. 

2) 3, since their power is 1 each by the time stage 5 happens.

3) As long as you use Path to Exile in stage 2 (or earlier in the turn), blocking the sliver will be legal since nothing will be preventing that when stage 3 happens. If you cast it during the declare blockers step (i.e. during stage 4) then it will be irrelevant because all blockers are declared in stage 3.