Sure, I can think of a few other formulaic approaches to practicing passagework. But the very best approach is not a formula at all, but to engage your mind analytically as you practice. You're probably doing this already and are just looking for some extra tools to have in your bag, but it's worth emphasizing. The most efficient practicing is not when we simply put ourselves through some process or ritual, expecting it to have an expected effect on our playing, but when we let our brain talk to our muscles and ears. "I do this. It sounds like that. Why? Oh, because I went like that. So I'll try going like this instead. It changed the sound in this way." This happens all the time unconsciously, but when we make it conscious we learn faster (and can apply our learning to other passages and pieces!).
With that disclaimer: I find that your "add-a-note" and "backwards-add-a-note" are very effective when in the early stages of "getting to know" a passage, helping to actually parse it and become familiar with the notes. Yes, practicing shifts in isolation is very important (though so is re-contextualizing them. Sometimes what makes a shift challenging might be what happened two or three notes earlier.)
One favorite, for passages that are all notes of equal duration (like, say, several measures of 16th notes) is to project a rhythm pattern onto them. To convert those 16ths into pairs of dotted 16ths and 32nds (long-short-long-short), and then to do the opposite (short-long-short-long). Those two combined do something great: If I were to apply it to, say, Paganini's 5th caprice:
... then with the first pattern (long-short), I'd have a "long time" between the A and the C (in which I shift), and a "short time" between the C and the E. As I move from the third beat into the fourth, I'd have a shortened time for the shift back to first position. When I switch to the short-long pattern, I'd have a "short time" between the initial A and C and a "long time" between C and E. By practicing both patterns, I will at some point have played every pair of notes, every shift and string crossing, at double my metronome speed (as if they were 32nds), but I give my mind time to process what I'm doing. You can then apply other rhythm patterns as well; Galamian's scale system has a whole section devoted to rhythm patterns (one 8th followed by three 16ths? A 16th, two 8ths, a 16th?).
Another simple trick is to change the bowing style. For a spiccato passage, the shortness of each tone gives your left hand fingers extra milliseconds to move into position, and a broad, connected legato forces them to switch places more quickly. For a fortissimo passage, there's much to be gained by calming the heck down and practicing it mezzo forte until it's comfortable. Sometimes for passages that have challenging left-hand work and bowing, it can be helpful to separate them. Put the bow down and just move your fingers on the strings. Then pick up the bow, keep your left hand still, and play open strings in the exact pattern of rhythm and bow directions that you would. For instance, in these measures of the Paganini...
... I find that the second measure is much more challenging than the previous ones, and is one of the measures that really impedes my ability to crank the tempo up. In that measure (starting with the F) I would play: D D A A | D A A A | D A A A | D A A A etc. I think it's those isolated D string notes, the string crossing from A down to D and back again, that's harder than a string crossing to an isolated higher string. That's partly because the Ds fall on down-bows. Try reversing the bow directions of the passage! If it started down-bow, start up-bow.
Many of these gimmicks do little more than shake up my routine and make me more aware of what I'm doing, but at their best, they cause me to stare at my bow arm as I slowly pivot from A to D and back, and contemplate the mechanics of how my elbow opens and closes as my forearm pivots, and consider whether it might be made easier if I bend my wrist like that, or if I bend my fingers like this on the up-bow to help curve the bow towards the lower string. And then I can internalize those physical findings and know why that measure is hard, and as I approach it think "Ok, bend the wrist and squeeze the fingers," and even apply the same lesson to a similar string crossing in another piece.
