How would you deal with trying to adapt the shadow spec to a non force user... Is there alternate ways to handle the talents? Should the mechanics stay the same but refluffed?

You can take the spec, Any talents that are force related do not work. If they are in the path of something you need you have to buy your way through but get no benefit from the force talent. You can't start with the Sentinel Career as that makes you force sensitive.

Alternately, you could pick the Sentinel:Shadow career, and just never purchase more force ratings or powers. The small amount of force you have merely powers some "intuitive" or "instinctive" abilities.

Lastly there is always the spy: infiltrator option. Or if we talk about supplemental specs, just getting the cheap force emergent / force exile, ignore the tree mostly and play a shadow who simply is force sensitive, but unaware of his use of the force like most force users are.