OK, I think I see what you mean now. Where the Reblitz book ("Piano Servicing, Tuning, and Rebuilding") describes how a grand's action works, having just explained what happens when you press and hold a key, and subsequently release it gradually, it remarks: "... when the front end of the key is less than halfway up, the action is ready for another complete cycle without the key needing to return to its rest position. When playing an upright, you must release the key and let it come almost all the way up before the jack slips under the butt, allowing the cycle to be repeated. When playing a grand, you can repeat notes quickly without waiting for the keys to return all the way to their rest position. Because it takes less time and finger motion to reset the action, notes can be repeated on a grand more rapidly than on an upright, particularly when playing softly."

This agrees with what you've been saying, but nevertheless my upright does not seem to behave in the manner above described; it reliably resets well before the key is even a quarter of the way up, perhaps it has an equivalent mechanism for enhancing repeatability, I can't get a good view of it to see. I reckon what limits my repeat speed isn't my piano, but my fingers, but I'll check if I can go faster on a grand, next time I get to one.
Good question. Could it be that when the una corda mechanism was first introduced, all notes had three strings? Another possible answer is that once the split approach of having one string per note in the bottom octave (or less), two strings in the next two octaves (or so) and three in the rest, became fashionable, that composers would have made UC/DC/TC instructions only where the material of interest lies within the three string range.
I don't see a problem there. Well, you know what they say. One ought to use the una corda pedal only to give a change of tone colour, not of loudness. Despite this, many (most?) players do use it as a "soft" pedal, and indeed it seems clear that historically that's what its main purpose was, any change of colour being a mere side-effect. The Beethoven examples I cited demonstrate that he intended use of the UC pedal to enhance both the crescendos and diminuendos beyond what could be achieved by touch alone, i.e. he sought greater extremes. No doubt many other composers also have volume foremost in their minds when writing the instruction (which is really only a suggestion) to use the UC pedal.

Insofar as that is the case, an upright's volume control capability is perhaps even superior to a grand's, not only because its effect is probably more pronounced, but also because it is continuously variable. In this respect (and maybe only in this respect), to paraphrase a popular song, anything a grand can do, an upright can do better.

Now, if you play something (a note, a chord, a section) forte without pedal, and then play the same thing again using identical touch but with left pedal (regardless of whether on grand or upright), then it will sound softer (never mind how much softer). To make something with pedal sound as loud as the original forte, you need to beef up your touch to the level which without pedal would give something approaching fortissimo.

Does the crescendo in your Bortkiewicz piece continue after the change to TC? If so, it seems plausible that the forte he writes just before the UC is not a "sound" forte but a "touch" forte, and that he may have intended the crescendo to be seamless across the change from UC to TC, in other words that he wanted the UC forte to be at the same level of loudness as the TC piano. Such a seamless crescendo is obviously easier to enhance using an upright-style soft pedal mechanism than a grand's una corda mechanism, because you would release the pedal gradually, not suddenly at the place indicated.

He most possibly meant a change of colour, not of volume. It begins pp (other pieces of his go as far as pp without ever indicating una corda.) In this piece, which is in a sort of binary form (the B section is really a coda 7 bars long while A is 16): A-A'-B. he begins A una corda at pp and reaches f, then in A' he asks for tre corde, beginning at p and progressing to ff. In B (or the coda) he returns to una corda at p and progresses to ppp. In The Butterfly, for example, he also uses pp and ppp but never una corda, while in The Angel at one moment he asks for ppp dolcissimo (una corda) for the repeat of the first section of A. Here I understand that it is tone colour he is after.

In the Grieg (to return to the subject of the thread!) there are many pp's and some ppp's but no indication una corda for those passages.