Any rule citation on that? What I find is the following:
Reference manual pg 2, Actions
* Each investigator is restricted to resolving each action once per round.
* An investigator must fully resolve an action before performin another action.
Reference manual pg 3, Component Actions
* Like all actions, each component action can only be performed once per round
The reference manual pg 11, Spell
No mention of casting only 1 spell per round.
If I am reading the rules right this means the player uses an action to cast a ritual spell and that spell must be resolved before any other action can be taken. However, incantations do not have the action keyword, although some are restricted by phase ("During the Encounter Phase . . ."), Thus multiple spells per round is possible - but you cannot interrupt a ritual spell with another ritual spell (those are all action based cards).
Also, you may cast 2 rituals in a round as long as they are not the same one. The spell section does say you cannot have duplicate spells, but each component action can be used once per round. It is perfectly legal to cast Feed the Mind as your first action and then Conjuration as your second action.
So the situation where you cast Feed the Mind, fail, choose 2 sanity loss, and then ask "can I prevent it with Instill Bravery" can legally happen - unless, as I said, there is a rule I missed buried somewhere else. If there is, I'd love the citation to resolve this issue.