Light shot was changed right? as far as enhancing Dia goes.
Correct, it was changed a while ago, now you can only shot once and it adds a flat 2.75% (still quite significant) potency to Dia, instead of the "increasing tiers" from before. So now Light Shot Dia II is "only" 23% def down and can't get higher, while Dia III becomes almost 29% down.
Correct me if I'm wrong but while the overall cap is +60% movement speed (and, as such, getting more than 60% speed from Bolter's Roll would be appearently useless, but one could say more than 48% if we consider 12% mov speed from various gear), there can be an use in Crooked Bolter if for instance you're under the effect of some debuff that reduces your movement speed.
C/G Boss in Sortie for example. If you're without the metal you won't be able to remove the Curse, and in such a situation getting more than 60% from Bolter's, can help getting closer to the 60% cap, since you start from negative movement speed instead of the typical 0%.
Niente da correggere, hai ragione.
Yeah that's right, lots of stats can get negative modifiers. The cap is generally global, where a strong Slow can be countered by stacking triple marches and/or geo haste. For movement speed, debuffs aren't the only reduction - most notably, Retaliation gives -14! Bolter's roll is definitely able to overcome that.
Note that some stuff has sub-caps, like weapon delay reduction, or Subtle Blow I and II. I have no idea how Monk's Roll interacts with stuff like Bruiser's Earring tho. I suspect it's just all additive and the cap is still +75% total - so you could wear 50 SB I and 35 SB II and Bruiser's Earring, and still be at cap.