I think they were fine as is.
Why can't inviability be used during battle? Why would something you absorb still hurt you? It wouldn't.
Plus the more powerful spells are balanced with the cost to make them
I agree you should be able to cast invisibilty in batte.
But i still think invisibilty was too esy to use,made they game easy.
What they need to do in my opinion is this.
Make it cost more magic
If you have no fatigue,the spell works but reduces time or transparancy.
Some enemies should have true sight.
Enemies can still hear or smell you,not all though. ( some have better hearing & sense of smell etc. )
Also another problem was the AI.....Example : If a mage saw you turn invisible,he could too,or be clever enough to use detect life etc.....or bandits could send dogs to sniff you out etc.
I believe AI response can help with spell balance too,if that makes sense.but i think all of these things would stop the spamming of invisibilty,and not make it so overpowered.