Ok I finally found where the issue was.
in java/org/l2jmobius/gameserver/network/clientpackets/Action.java
commented out this line:
// Check if the target is valid, if the player haven't a shop or isn't the requester of a transaction (ex : FriendInvite, JoinAlly, JoinParty...)
/*
* if (player.getActiveRequester() != null) { // Actions prohibited when in trade player.sendPacket(ActionFailed.STATIC_PACKET); return; }
*/
Don't know if i'm breaking something else but it now allows me to attack while i'm being invited to a party or offered a trade.