Morale is fine.
I'm getting used to the general tone of this commentary - and that's what they meant. The repeated attacks will weaken the resolve of the herd to resist. OK so the herd will become increasingly jittery as a result of the repeated attacks, but spirit? morale?
Yes, as in English, go for spirit/morale.
I suppose the "spirit" which is weakened is the determination of the herd to carry on to wherever it is going when it encounters these lions.
Would it be out of order for me to ask if you have bitten off more than you can chew in the task you are undertaking at present? - not that I mind the queries, but I would have to wonder at how fluid the commentary will be if you are so uncertain of the meanings of so many words. How much leeway do you have to to stray from the original script? It might be better if you wrote your own script for what you are seeing, referring to the original only for guidance. The script you are working with appears to be full of wildlife documentary cliches and meaningless waffle (probably there to pad out the action on screen).