Rock and a hard place
Unfortunately, all the really good spells that would help, Zone of Truth, Commune, and so forth, are purely in the realm of Cleric. And for those that aren't, no one is gaining a level to swap out spells.
There is a long shot...
Drop a couple of Detect Thoughts scrollscrolls somewhere the party will find it.
For the duration, you can read the thoughts of certain creatures. When you cast the spell and as your action on each turn until the spell ends, you can focus your mind on any one creature that you can see within 30 feet of you. If the creature you choose has an Intelligence of 3 or lower or doesn't speak any language, the creature is unaffected.
You initially learn the surface thoughts of the creature--what is most on its mind in that moment. As an action, you can either shift your attention to another creature's thoughts or attempt to probe deeper into the same creature's mind. If you probe deeper, the target must make a Wisdom saving throw. If it fails, you gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates). If it succeeds, the spell ends. Either way, the target knows that you are probing into its mind, and unless you shift your attention to another creature's thoughts, the creature can use its action on its turn to make an Intelligence check contested by your Intelligence check; if it succeeds, the spell ends.
Questions verbally directed at the target creature naturally shape the course of its thoughts, so this spell is particularly effective as part of an interrogation.
The wizard can scribe the spell from one scroll overnight into their spell book. Next day, they can quiz all the other party members and learn the truth.
But what about the Wizard? How do we trust them?
Since the spell is also available to Sorcerers, your Sorcerer can use the extra scroll(s) to quiz the Wizard without issue.
Last hurdles
There are two potential problems:
- There is a saving throw so it may take some time to get all the party questioned. There is no limit to the number of tries (you're not immune to the spell, just this casting), so if someone saves, try detecting the thoughts of another character. Then try the first character with the next casting.
- Will the party trust the results? Or are they too far down the rabbit hole?
