Nice idea to change assassins to be more interesting to use during all game ( i never play with them because servant thing, and their bug to lose all bonuses for find weakness when you remove them from you`re units inventory - like going through wizard tower, you pick some other unit, come back to castle and pick them back and ALL their find weakness is back to 0%
)
As for removing debuffs check archdemons and their passive talent Restoration that remove all negative effects before their turn , if manage to find where it`s described how it works
For evade % find how it work on sea dogs - agility talent.