JUST TO SUM IT UP( ALSO BECAUSE TALKING ABOUT INITIATIVE AND BARRIER IS MY FAVORITE PASS TIME)

Han's mystery description says it removes shields and buffs. Can someone explain which type of buffs it can't remove?

BUFFS are anything that is mentioned in description as lasts for certain number of rounds. So just as you mentioned iruka,s, mifunes, guy's buff are all removed.-

AURA is a buff that lasts until the person who provides it survives like poison tai, water mains healing buff, wind blade asuma's buff,

BARRIER is NOT a BUFF. It can only be overlapped by your own barrier or be destroyed by roshi, tendo etc.

1) Can it remove barrier buff? yes or no, plase explain why if possible.

NO

2) Main passive, like lightning main's anbu tactics (25% tai boost) or earth main's defence knowledge , these skill description says it works as long as main remains in field. Can han remove it?

NO because they are AURAs.

Does this same apply to water main's healing boost passive too?

NO because again that passive is an AURA.

3) Hashirama passive (ninja god) gives 30% nin boost for 5 round, Han can remove it. But with skillbook, it can last the entire battle duration. Can han remove this passive now?

I am not sure about hashirama stuff(not lucky with those ninja packs), but i have heard the same thing as you mentioned that with skill books hashirama passive moves from BUFF to AURA.

Anything else related to this, if you want to mention/explain please do so.

Regarding scaling up attributes like scaled up defense and ninjutsu for roshi on receiving damage.(little bit doubtful about this). I have heard that when Han does the first part of his mystery these scaled up values get resetted back to base attributes.