Some cautions

First, isn't there some other way to use this 'extra' length near the slip?  Or, can't the RR negotiate a new lease?

I did this with Atlas TO's... with success... after some practice!  What I learned ...

The #1 hazard is stressing the 'spikes' that retain the rails... and leaving too few along a rail segment.  For practice, I used Brass parts as templates, and 2nd-hand turnouts to warm up.   ON AN ATLAS turnout, I needed at least 2, preferably 3 full ties from any frog or moving part.  I could shorten most turnouts by about 1.5 inches.  I began by trimming the ties from the underside using a razor blade, and sliding them off the rail ends. Then I trimmed the rails.  I also removed those partial ties (with clearance for joiners) and replaced them with (a pair of) normal ones.  Tie spacing was sometimes sacrificed here for support at the ends.  Adhesive (ties to rails) is advised.

When finishing the rails, Do NOT file the rail ends... use a dremel and a stone!   File action can grab the rail and rip it from the ties!  But first, clamp the rail to the ties with a wood or masonite cover.

In your case, I'd first seek the  cheapest available turnout by Walthers... to get a feel for their manufacturing tolerances.  Can you, in fact, slide the ties off... or would you need to peel them away? And, what needs to be done to maintain the gauge while working... if the railheads lean in, the 'spikes' are at risk!  (see my thoughts at the end.. I may have 'solved' this!)

In your case, the sheer expense of an error is a bit daunting!  I might try clamping the entire slip into a sandwich of masonite... exposing the length I want to trim.  Then I'd proceed as above.  

(just came up with a thought... solder a length of PC board across the railheads where you intend to trim before any work for support, then work from below: trim ties and cut rails with PC board.)

Good luck !

Walthers turnout shortening

With the double slips it looks like you will need to leave 4 or 5 ties after the frog at least.  I don't think you'll have any problems as long as you trim carefully and keep everything aligned afterwards. 

I just did something similar with a pair of Walthers #8 turnouts.  To create parallell tracks on a 2" center I needed to trim the branch side of both turnouts.  I aligned the turnouts approximately how I wanted them and had the areas that needed cutting overlapping each other.  This gave me an idea of how much trimming was going to be required and where.  I then cut through the rails using a cut-off disc in a motor tool and slid the pieces out of the end, leaving all the ties and spikes intact.  I then overlaid the turnouts again and marked where to trim the ties.  I can take a picture of the end result when I get home later if it would help.