Gameplay in FPSC currently is linear only in sequential order of level numbers - trying to achieve anything else would be asking for major problems.
Again as I have said previously regarding other issues - this one concerning directional movement through game levels has been asked for to be considered for improvement in a variety of helpful ways which I need not detail here. This I can say as I asked for consideration to this end when beta testing and it remains in a long list of requests.
Again TGC are aware of the point or issue as they are to almost all others you can think of.
Unfortunately perhaps, though I cannot comment on TGC thinking as I am unaware of it - but as a general comment or assessment there a vast array of requests that users have made that will probably never see their way into FPSC. Perhaps I am wrong but I just dont see it happening not at least unless something radical happens and FPSC slowly or miraculously overnite becomes a best seller and then perhaps anythings possible.
Just my opinion and probably severely flawed.
Sorry I am not sure if Riker9 has anything to add to the level movement issue.