1; yes, but only on a creature attack, so the dwarves would have to have this applied to their scratch or punch attack. Can't apply specialattack to items.
2. Not really. Stemming from an interaction, a syndrome doesn't know what body part was hit by the attack that started the interaction, and thus you would have to tell it to target, say, the upper and lower bodies and head and limbs, and from there you can apply bleeding, pain, and loss of function syndrome effects. You could also lower the creatures attributes like toughness which would make it more vulnerable to physical damage. But I don't think there's a way to directly damage tissues i.e. create a wound through a syndrome/interaction except through necrosis. Reducing toughness is a fair bet for inflicting more injuries, but increasing the strength and agility of the dwarf with an on-self defend CDI will be just as reliable.
3. No, with the exception of interactions applied through specialattack_interaction, like werebeast curses.
4. Necromancers appear to apply their interactions, but currently custom secrets never build towers because it's become a hard-coded behavior associated with generated necromancer secrets. Free roaming secret-knowers or creatures that have access to a CDI do not appear to ever actually use CDI interactions they know, for instance dragons and fire imps don't burn people in worldgen, but effects applied to them by the secret, for instance transforming them into a different creature, do get applied so an interaction that increases their strength should technically be taken into account during wg fights.
Interactions don't work off-map during fort or adventure mode any more than they do during WG, either. Creatures can only use CDIs while loaded in play
The wiki isn't clear on a lot of this stuff, so asking questions is important as actually writing interactions and getting experience with them. I've considered writing an article about how to actually use interactions and syndromes, but I am both busy and lazy.