[Mardagg]

The first 2 questions I can only guess:

Injuries remain for the same round,I would assume there the same rule as with several battles in one province(for example nation A attacks independents and in same round nation B attacks nation A in this province).

Fatigue is canceled,likewise the rule,when several battles occur.

In general I find Assasins quite useful.
I use them a lot for early expansion,if and only if I play Machaka or Abysia.
In case of Machaka,the Assasins are expensive and capital only,but come already well equipped.
They can beat even Knight commanders about 75% of the time.
You can make one your prophet,especially if you have a 9er blessing,and you get an early SC,who will beat every commander in provinces with your dominion.
I use scouts or neutral commanders to scout provinces about number and quality of commander and write them down.
Knight provinces with lots of knight commanders or provinces where indie commanders have bodyguards I either try to take with an army then or as soon as I can give some items to the assassins.If its a very good/important province,I often take the risk with Machaka Assasins.
Even if they die,the knight commander mostly will be killed,too,because of the bane blades.

In case of Abysia,I never equip my Assasins in early game.Thus I never try to assasinate difficult commanders.
Normal commanders are quite easy though.
I specialise on provinces with only 3 indie commanders,preferable one of them a priest,and then send 2 slayers there(if one dies I still have another).Since slayers are not that expensive and can be build in all your castles,you can quite early capture quite a lot of indie provinces this way,if its a big map.

Very important for Machaka and Abysia is,that you place your Assasins such,that they will do the first strike.
This is just a matter of experience,since most independent commanders start always at the same position during an assasination attempt.
Its important here,what kind of commander you will face and the movement points your Assasin has.

Later on assasins become less useful,but there are still worthy scouts and can take down small defended provinces of the enemy.
In case of Machaka or Chuzrael,you can use them for a lot more,though,if right equipped.