If an ADMINISTRATOR punished a player, then a MODERATOR can't revoke this punishment,
If a MODERATOR punished a player, then an HELPER can't revoke this punishment but an ADMINISTRATOR could do it

+ create permissions bm.command.[unbanip/unwarn/...].own for each punishment type

So the first request isn't possible. Bukkit's permission system does not allow for checking permissions of offline players.

The second is, I can add .own permissions for unbanning ip, dwarn etc

For the 1st, you could create a system of personalized permissions like bm.staff.<rank>
=> bm.staff.administrator / bm.staff.moderator / bm.staff.helper, then when someone is punished, the <rank> is stored in the database

In config you :
+ add the possibility to enable/disable the feature
+ create a category to define the rank order like :

staff-order-revoke: true

staff-order:
  - administrator
  - moderator
  - helper

and then if this feature is enabled, when someone wants to revoke someone's punishment, you check in the database the rank of the punisher compared to the actual bm.staff.<rank> of the player who wants to revoke the punishment

+ then it could be interesting to add the option to disable the possibility to staff members to revoke punishment applied by others staff members with the same rank :
like MODERATOR can't unban a player banned by an other MODERATOR

For the 2nd, it's exactly what i meant ^^

How about relying on Vault to allow permission plugins that support checking offline players?