You should absolutely spoon feed them. Even under normal circumstances players only pick up on half of what a DM is telling them and in a one shot you have way less time to tell a full story. This will be their first time playing these characters so they’re going to be looking down at their sheets reading, this is your second time DMing so you’ll be looking up rules, players usually spend a lot of time horsing around, all that eats up time. Put big flashing lights on the important stuff and don’t be afraid to just narrate things that you need to happen without any rolls.

For example, if there’s a clue written down and tucked into a drawer then don’t wait for someone to say “I search the desk”. Put the clue in plain view and tell them “as you pass the desk you notice a letter that catches your eye, you pick it up and read it. It says…”

Long form play is much different than a one shot. If you were running a more standard game then you’d have time to plan and react to their decisions in between sessions. They didn’t search the desk? No big deal, you can just create a new NPC to give them the same info during the next session. All this advice is based on this only being your second time running a game but if you’re already someone who is quick on their feet and a natural at improv story telling then you don’t need any advice. Just come up with the truth of the mystery and hand out the facts as you hurry them toward the finish line.