I think the shed is just an extra store for items to free up slots. In the planning phase you organise your inventory's with what you will need and leave bits you will need later in the shed.
It is useful in solo play where just holding the materials to build buildings can make it hard to keep items for gifting, for selling, for building stairs and for possible bundles. Like many things in the game, by the time you get them there's not much year left to make them overly useful most the time.
I think this would be abuse of the card so you would not do that. The technical answer already given that roll implies only used when rolling also removes abuse of the item. Pick 2 other spaces also implies you can't pick the same space, but for this I think you are okay to pick another monster space, but you would not use the slingshot again; you would resolve any combat you choose or invoke another card where possible.
Already answered but the card should say 'during your turn' or 'spend an action to' if those are required, otherwise you would do them immediately I think. Also look out for the gift icon, for those you keep the card and the stuff happens when the gift comes up on a season card.
If it does happen immediately it's safe to assume the card is also discarded afterward because there's no other way to invoke it. If the card is on gift, on turn or spend action, I would assume to keep the card unless it states to discard it.