I think fiber cement siding as a class of materials is excellent.
One has to learn to work with it.
I think pre priming is a good idea and suggest painting cut ends.
As for paint sealing the house, I can offer this experience:
Our weather board / lap siding / clapboard was painted and all the
joints caulked closed.
( This was a "gift" from the painter who wanted to do a "good" job.)
The paint blistered.
We opened every third lap joint and the problem has not reappeared.
I think good prep work will take care of the sealing up of joints.
TB