3rd thing only happens if you don't have other spells that you picked up. You can only add spells to your deck if you don't have enough spells in the spell pool to fit into your deck.
If you or anyone else acquired spells during the scenario, they will be in the spell pool at the end of the scenario and you will have to add one of those to your deck.
Remember, nothing is stopping you from slotting a single Cure into your deck as an Arcane caster and then passing it off to a Divine caster to play. You have it as an "oh crap" moment if you aren't next to them and need to heal yourself, or you can pass it off at the start of your turn and let them have an extra Cure for the scenario, useful if one of them winds up in the discard due to a failed recharge check.