I would actually put a different spin on it, that our will is perfectly free when there is the most basic presence of sources of temptation. and that when sources of temptation are intensified, it makes us less free, because the more difficult to overcome sin, the more likely we will choose to be in bondage to sin. without an intense temptation you have a 100% perfectly free choice: do you want it or dont you; no pressure. with intense temptation there is now extreme pressure to sin. with pressure to sin, your ability to choose freely is hindered only in the sense that it makes it harder to make a perfectly free choice.
There is no violation of free will because the choice to sin is still 100% there no matter how hard or easy it is to sin. free will is simply the ability to choose between what you want without being forced by another agent to choose it. so if someone holds a gun to your head and demands you to do something, even in this instance your free will has not been violated: you can refuse to obey the commands of that maniac gunman. but with a gun to your head it makes you much more likely to give in to the gunman's demands. so the gunman hinders your ability to make sound choices. but it does not disable your ability. if airplanes and cars and boats stopped working, it wouldn't prevent your ability to travel to another country, it was just significantly hinder it; thus so long as you still have the freewill/ability to travel to another country, you still have the freewill/ability even though its been hindered by the obstacles being presented against you.
as to why he doesnt' make it harder to sin now rather than later, i would say its because as mortals, our nature is such that we cant have a 100% unhindered type of choice nature. because being mortals there will be various pains we have to go through necessarily and we will have to suffer from the fact that we are ignorant and limited, and that we have to work harder to get the good things we want. the angels being immortal have much more knowledge, ability and intellectual capacity that make them better to do their jobs. withi not having as advanced bodies/natures as the angels, we are by definition more susceptible to temtpations than they are. this is solely because we are mortals. if GOd had not made mortals then we would have had a much easier time to choose the right thing. so a better question might be why did God make mortals in the first place.
However, you did mention that "the gifts and callings of God are without repentance". 
