I had another idea. It's bloodily expensive, but it can work, using Glyph of Warding in a new, creative way.
The Glyph of Warding spells has a spell option. According to the wording of the spell description, "You decide what triggers the glyph when you cast the spell." Example of the most typical triggers are given, but it doesn't limit the trigger to the most typical. Rare, creative triggers are possible, and decided by the Wizard. Among the most typical examples, "approaching within a certain distance of the glyph" is mentionned, meaning that the trigger can happen anywhere, as the "certain distance" isn't limited. The spell stored within the glyph is limited, but "if you create a spell glyph, you can store any spell of up to the same level as the slot you use for the glyph of warding" and "the spell must target a single creature or an area". It is explained that ". If the spell has a target, it targets the creature that triggered the glyph."
What is a target? Sage Advice adopts a very broad defintion: "When the rules say "target" they really mean the English definition... The meaning that the rules are getting at is that when you choose someone or something to be subjected to some kind of effect, that's one of the common ways the word targeted is used in English... Any time a spell is telling you to pick a creature or an object or a point in space, to be affected by something, really that thing is functioning in that moment as a target of some kind."
According to this definition, Gate targets a hapless creature to summon it. Therefore, a Gate Glyph of Warding, triggered by "whenever someone casts Meteor Swarm at a Wizard on the same plane of existence", would pull any would be assassin to the Glyph of Warding. It's an abjuration, so obviously wouldn't be countered by spells that protects from divination. It could lead to false positive but 9th level spells are rare, Wizards assasssinations are even rarer.... If the killers' modus operandi was confirmed, it would be a good way to trap him.
The Teleport spell normally targets the spellcaster and up to 8 willing creatures, sending them to a location, with a possible chance of error, 0% in case the spell is cast with an object associated to the destination. A creative Wizard could take a root from a grove (let's have this battle outside of inhabited places) a create a 7th level Glyph of Warding, storing the Teleport spell to the secluded grove, using the trigger mentionned as above. Then tasks his Simulacrum to keep his action ready to Feeblemind any creatures that could appear. Some could object that in English, "must target a single creature or an area" disqualifies spells that targets a single creature and tag alongs, making Teleportation ineligible. This is debatable and the lack of DM agency would let this work.
In case it's not possible, an even funnier spell to store within the Glyph is... Simulacrum. It explicitely targets a single creature. So, whenever the trigger is activated, poof a Simulacrum of the offender is created, loyal to the spellcaster. Obviously, the best way to set it up for minimal inconvenience for the wizard is to memorize Simulacrum and Glyph of Warding among others, cast Simulacrum expanding an 8th level spell slot, so the created Sim has both 7th level spell slots and Wish, then have the Simulacrum expand his two 7th level slots to create the Sim-generating Glyph of Warding, and stand idle waiting for the trap to function and warn the Wizard when it triggered. This is now Alfred's preferred method, if he decides the threat of the Cheating One is enough be of concern (which would need him to have a valid build as a prerequisite).