going to take a look at your link in a moment.
following is just a thought, not sure that i'm correct. not sure precisely where your pad is separating, so i can't be sure that the following is not totally "off-the-wall". do me a favor, please, think about it and if you think i've erred here, let me know.
>>"unrelated to modifications"
perhaps only partly true. certainly use/wear could be some percentage responsible (don't know if these pads, unmodified, experienced this problem). the mod, however, causes a higher delta-P (pressure difference b/t no use and in use) to result when laying on the pad due to the decreased volume inside the modified pad (viz., Boyle's Law). i'm assuming here the majority of the weight of the upper body is still exerting force on the pad - legs/feet more minor contributing factor to a pad's "in-use" internal pressure.
furthermore, due to smaller internal volume, the higher pressure on the pad's outer mat'l, pushes it "out" more forcefully. that may be causing it to separate from the foam. what do you think, am i getting this right? or, am i all wet? -- been known to take a good "dunking" every now and again.
issue would be what pct of pad's vol. was removed and what pct of total body wt is still exerting press. on the pad. if pcts were equal, then no diff in press. resulted from the mod. however, if 80% of body's wt is still on pad, and your pad is now only ~60% as long, then a higher in-use int. press. results. perhaps, this exacerbated the situation slightly and caused the problem to occur a bit sooner and progress a tad more rapidly. the problem would obviously occur where attachment was weakest. that may not be near the area of modification. might be due to perhaps small inconsistencies in initial construction/manufacturing???
furthermore, let's just say ALL of the "separation" was cause by the mod (not saying it is, let's just assume), then the separation may stop at some point due to the increased pressure stresses being relieved by separation. was there any separation prior to the mod? if no, this does NOT necessarily mean that the mod caused all of the separation. your pad has also experienced more use, under which it might have separated even without the mod. guess we can't know for sure.
how much should be attributed to use and how to the mod, i certainly don't know.
looks like a very nice job. nice pics. good write-up. also, nice disclaimer ("cat's ear" - hadn't heard that before). did you decide to pack it inside your sleeping bag? if so, have the edges/corners damaged the bag's lining any? again, nice job.