Did some more research by comparing a spawned unit in arena mode with the secret (interactions work ok) with one that received the syndrome.
The wound is properly created (in 'body') by DF when the syndrome is added. It contain a curse info with all the proper data, said curse is then applied to the unit.
The only divergence was the lack of a reinfection id and count, but after modifying syndrome-util to add it, nothing changed.
Everything in body, in curse and in syndrome is adequate. I'm still searching...
Quoting myself, but after figuring it out I believe you might be interested.
Dfhack wise, the syndrome is properly added as far as interaction goes. But Kruggsmash made a video about turning dwarves into necromancers so I knew that it had to work.
I tested with an inhaled stone that provides interactions and the units were not using it either, so taking inspiration from generated necromancy I removed usage hints and it worked.
As it seems, ATTACK is not working anymore, but no hint now has the same effect (instead of "all the time" from 0.34) and the unit will use the interaction in combat. The arena does not respect those rules, so it's probably a bug. Mystery... kinda solved, I guess?
[INTERACTION:HELLFIRE_SECRET]
[I_SOURCE:SECRET]
[IS_NAME:the secrets of hellfire]
[IS_SPHERE:FIRE]
[IS_SECRET:SUPERNATURAL_LEARNING_POSSIBLE]
[IS_SECRET:MUNDANE_RESEARCH_POSSIBLE]
[IS_SECRET:MUNDANE_TEACHING_POSSIBLE]
[IS_SECRET:MUNDANE_RECORDING_POSSIBLE:objects/text/book_instruction.txt:objects/text/secret_fire.txt]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_REQUIRES:CAN_LEARN]
[IT_REQUIRES:CAN_SPEAK]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[IE_ARENA_NAME:Pyromaniac]
[SYNDROME]
[SYN_NAME:Pyromaniac (fireballs, directed ash, firejet)]
[SYN_CLASS:MAJOR_POWER]
[SYN_NO_HOSPITAL]
[CE_DISPLAY_TILE:TILE:64:4:0:1:START:0]
[CE_DISPLAY_NAME:NAME:pyromaniac:paromaniacs:pyromaniac:START:0]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Breath flames]
CDI:USAGE_HINT:ATTACK]
[CDI:INTERACTION:MATERIAL_EMISSION]
[CDI:BP_REQUIRED:BY_CATEGORY:MOUTH]
[CDI:FLOW:FIREJET]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:MAX_TARGET_NUMBER:A:2]
[CDI:WAIT_PERIOD:120]
[CDI:VERB:exhale a cloud of fire:exhales a cloud of fire:NA]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Hurl fireballs]
CDI:USAGE_HINT:ATTACK]
[CDI:INTERACTION:MATERIAL_EMISSION]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:FLOW:FIREBALL]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:MAX_TARGET_NUMBER:A:2]
[CDI:WAIT_PERIOD:30]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Raise cloud of ash]
[CDI:USAGE_HINT:FLEEING]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:INTERACTION:MATERIAL_EMISSION]
[CDI:MATERIAL:ASH:TRAILING_DUST_FLOW]
[CDI:VERB:conjure a cloud of ash:conjures a cloud of ash:NA]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:MAX_TARGET_NUMBER:A:1]
[CDI:WAIT_PERIOD:90]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Spray jets of fire]
CDI:USAGE_HINT:ATTACK]
[CDI:INTERACTION:MATERIAL_EMISSION]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:FLOW:FIREJET]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:5]
[CDI:MAX_TARGET_NUMBER:A:2]
[CDI:WAIT_PERIOD:150]
[CDI:VERB:conjures a jet of fire:conjures a jet of fire:NA]
It is added with :
local synName = 'Pyromaniac (fireballs, directed ash, firejet)'
dfhack.run_script('modtools/add-syndrome', '-target', unit.id, '-syndrome', synName, '-resetPolicy', 'DoNothing')
That does not explain issues with bleeding or drowsiness. Those symptoms are a little complicated as those counters were not in body, probably in body parts. Again, it is worth testing it with inhalation before going further.