How to reduce total screw-overs in Ticket to Ride so there's always a potential path toward victory?
In my opinion, there shouldn't "always be a potential path towards victory".  I've never found it a big problem to get my routes finished - though obstacles and impediments are often flung in my way!  There are a few things you should be doing to ensure that you always have the best chance of success, though:

(1) Obviously, don't overcommit.  Know how far 45 trains can actually get you.  Also, when calculating how many trains a route is going to take you, don't count up the optimum route.  Leave yourself plenty of wiggle room if a colour you were banking on doesn't become available... or worse, if another player starts building routes that you were planning on being the backbone of your strategy.

(2) Don't start building too soon!  My wife often does this, starts methodically building her cross-country route from point A, slowly and steadily towards point B.  This only succeeds in giving me plenty of warning where we are likely to come into conflict in the middle.  As such, I usually have plenty of time to save up for and grab those routes, she gets mightily teed off, and I get to sleep on the couch YET AGAIN.  But really, if she didn't telegraph her plans, I wouldn't find it so easy to thwart them!  

Obviously, you can't fall too far behind the other players' builds, you don't want them to run out of trains way ahead of you.  But extra information is often a lot more valuable than being first out of the starting gate.

(3) Conversely, if a route really is essential to your plans, don't hesitate, grab it straight away.  This is particularly important in variants like Nordic Countries, where there are several key one-train routes linking the left and right halves of the board.  It's no accident that you get dealt some train cards BEFORE you have to choose which routes to take on; quite often I will only keep a route that relies on getting from one side of the board to the other if I have the right colour to snap up the link in my first turn.  Don't take risks on the really important stuff.

In general, I think you may be overvaluing routes.  Huge cross-country routes are worth a lot of points because they are hard to pull off without a hitch!  One of the players in my group has what I think is a possibly better, and certainly funnier strategy: she takes a few short, simple routes at the start of the game, that can't possibly give her any trouble.  She builds those and as many random obstructive long routes as she can, bringing the game to a speedy conclusion.  Very funny when this results in people getting -50 instead of +50 points, because all their destination tickets were too ambitious in the light of this playing style...

I know we've said "Ticket to Ride is a well-balanced game, don't mess with it", in response to your questions before, and you haven't really appreciated it, but I really do believe it.  There's more going on delicate-game-balance-wise than I think you appreciate, and I think you should try to learn to love it for a bit longer before you tamper with it!