If a pc readies SHIFT (acts like Dimension door, SU ability) against the approach of an enemy NPC who has not yet moved their full speed and shifts out of their path (to the side or behind the npc) would it cost the npc any speed increments to turn and face the new direction?
Each round is 6 seconds, which in terms of pivoting (or close quarters fighting in general) is a long time. The NPC is also considered to be looking in all directions while moving toward you, wary and ready to adjust to the situation. So Shift, by both rules and reason, isn't going to do much for you if they have enough move to get to you.
It might work if they're flying and can't land or maneuver well enough.
Arguably, there are conditions they might have or situations where this tactic works better, but I think a Reflex save in most situations is overly generous. Just imagine how player would feel if their PC had to make a Reflex save because an enemy repositioned like this.
This would work with charge because they're committed to a full-round action, and you're gone. Also works because you're doing a defensive action rather than an offensive one (which would occur after the charge).
One way around this is to ready for when they attack (assuming your PC can sense the attack before executed). Then they're committed to the action, and since your defensive ready goes off first, you're gone and they have no actions left to get to you.
No because the opponent's movement action hasn't finished.
If a pc readies SHIFT (acts like Dimension door, SU ability) against the approach of an enemy NPC who has not yet moved their full speed and shifts out of their path (to the side or behind the npc) would it cost the npc any speed increments to turn and face the new direction?
There is no facing so no penalty/cost to turning around. Starfinder and Pathfinder share this concept.
Even when not your turn your character is aware of what is happening all around them.
Doing as you suggest merely puts you adjacent to your enemy.