Rav Chaim Kanievsky quotes the Mishnah Berurah:
טלית שיש לה יותר מד' כנפות כגון בת ה' ... חייבת בציצית ... וי"א דהוא מדאורייתא ויש להמחמיר אם אפשר אבל אם כבר נעשה בכנף הקרוב ואי אפשר לתקן ... יכול לסמוך על המקילין ולברך.
So some say that the requirement to keep them furthest apart is a Torah matter, so it is preferable, however if it isn't possible, the lenient opinion can be relied on, even to say a blessing.
Applying that here, it seems the garment's tzitzis in your case would be Kosher according to the main opinion (since even if the fifth corner remained he relies on it to say a blessing) as removing the fifth corner isn't going to suddenly make it worse.
In and of itself, that answers the main question (the issue with the furthest corner according to the controlling Halachic opinion is that this is not an issue which invalidates the Tzitzis, so if it was Kosher with a (non-fringed) fifth corner there kal-vechomer without the fifth corner there), and this opinion can be relied on to say a Bracha if other options are not available.
However the question remains according to those that say the Tzitzis were posul, so that fourth corner was never obligated in Tzitzis. It seems to me that it is the logical equivalent of putting it on the side and then cutting the garment down to make that a corner. Such a case is like putting the Tzitzis further in than three thumbs from the proper corner and then cutting a hole to move the Tzitizis to move them closer. If you do that, it is considered מין העשוי. To quote the Shulchan Aruch Harav:
ואם עשה הנקב למעלה מג' גודלין ואחר שהטיל הציצית בנקב חתך בנקב כדי שיתלו הציצית למטה מג' גודלין פסול משום שנאמר גדילים תעשה וגו' ולא מן העשוי כבר בפיסול
If he made a hole higher than 3 Thumb-widths and after he tied the Tzitzis in the hole he cuts the hole in order that the Tzitzis hang below three thumbs it is invalid ...
The description of the problem of מין העשוי from more at length in Shulchan Aruch HaRav:
שמתחלה כשהטיל הג' ציצית בבגד היתה עשייתן בפסול שעדיין לא היתה הטלית מחוייבת בציצית כלל שהרי לא היה בה עדיין ד' כנפות ואע"פ שאח"כ עשה בה גם הכנף הד' מכל מקום אין הג' ציצית הראשונים חוזרים להיות כשרים על ידי כך כיון שלא עשה בהם עצמן שום מעשה חדש להכשירן והתורה אמרה גדילים תעשה ולא מן העשוי כבר בפסול
Originally, when he placed the three Tzitzis on the garment, they were made invalidly for the clothing was not yet obligated in Tzitzit at all for it still didn't have four corners, and even though afterwards he made a fourth corner, nevertheless the three Tzitzis don't return to being valid through this since he didn't do a any new action to validate them, and the Torah says "You shall make fringes" and not from what was already made invalidly (emp. added).
Based on this the problem of מין העשוי applies when you make a Tzitzis in a place where either the whole garment does not (yet) require them or on a garment that does require them, but not the place that requires it, and do no further action on the strings to make them Kosher. According to the opinion that the Tzitzis are invalid on the non-furthest corner, that one was not made in a place that required Tzitzis (according to that opinion), so it is מין העשוי when you remove the 5th corner.
