Tom,
I'm not going to tell you that you're wrong. But neither am I going to roll over. Regardless, it seems to be rather well known that Mahler's piano 'chops' weren't in the best shape when he made those rolls. Also, the allotted time was quite short (or so I've read). In addition, I've heard from fairly reliable sources that the mechanical works of the piano - whatever it was they did - wasn't the easiest to work with either. Mahler may have walked in a sort of 'hiccup' manner, but I somewhat doubt that he always conducted that way. In addition, I don't feel that indicated glissandi has ANYTHING to do with degrees of tempo modification. And, by the way, many modern recordings do, in fact, observe the written glissandi. Added or implied glissandi are, of course, a matter of interpretive license.
On top of all that, I fail to see how Mahler's piano rolls are somehow a vindication for the sheer amount of extreme contrasts in tempi that Mengelberg 'peppers' upon his first movement to M4. I'm not saying that Mengelberg was somehow wrong, but I'm not going to 'buy' that that's how Mahler did it himself, without some real proof that that, indeed, was the case. If one thinks about it, what would have been the point of Mahler and Mengelberg 'copy catting' each other during that famous concert where they both conducted M4? Wouldn't that have been kind of boring? Wouldn't a concert where two conductors 'copy cat' each other sort of go against the very types of arguments that historical buffs tend to make?
Anyway, that's just my zwei groschen, as it's ALL a matter of speculation without hard evidence either way (as I already stated).