Can I use multiple items and spells in one action?
In the action phase you can perform actions that are prefixed with the text Action: or Local action: on any of the cards that belong to your investigator. "Component" is just a very general term for all these cards. There are so many of them, it would be tedious to list them all. These are items, artifacts, your investigator card, and so on.

The rule you quoted at the top is the explanation of how to handle component actions in the framework that you can do only two things in a round and cannot do the same thing twice. For example if you have a reusable artifact with an action to damage a monster on your space, you could use it only once per round. After that you will have one more action left.

In your post I don't know what you meant by this bullet point list in the action phase:


Feed the Mind (ritual spell)
Blessing of Isis (ritual spell)
Dynamite (item - weapon)
Holy water (item magical)
Travel


You can do any 2, and only 2 of these things in the same round.

As per preventing, you can use bandages to prevent damage right at the moment when the damage would be dealt. If the card says you can use it only in combat encounters, then you cannot use it while resolving a clue or other effect, it applies only in combat.

As for using something to help other players, you can only do that if the card says it's a local action or explicitly mentions. If it's a local action then you can only use it on players on the same space as yours.