For context our party used a bean from a bag of magic beans that created a Mummy Lord (ML) and its lair, we are all at level 5 and a Mummy Lord in it's lair has a challenge rating of 16.

There is no chance for us to take the ML out by any normal means but the idea I have for achieving a victory revolves around the title.

So I am curious as to whether a familiar summoned by my character (a wizard) would be attacked by the undead within the lair or the ML itself if it enters the lair (assuming my character himself is at the furthest possible distance he can be from the lair).

Each undead creature in the lair can pinpoint the location
of each living creature within 120 feet of it until
initiative count 20 on the next round.

Given that the above lair ability seems to be combat related as it only lasts one turn, I have the impression that I could avoid a combat encounter with the ML by using my familiar as a spider and the casting glyph of warding (GOW) through the familiar to setup a trap for the ML (Spells with a range of touch can be cast through a familiar as though you were casting them).

So my plan that I have been thinking of was to use the GOW through my familiar but infuse it with the fireball spell. After doing some calculations based on the ML stats and such I came up with a minimum of 80 damage from 10 GOW infused with the fireball spell.

Each creature in a 20-foot-radius sphere centered on that point must
make a Dexterity saving throw. A target takes 8d6 fire
damage on a failed save, or half as much damage on a
successful one. The fire spreads around corners.

The 80 damage comes as a result of assuming that the ML succeeds on every saving throw (it has a resistance to magic so has advantage on all saving throws so this is not unlikely) and so takes half damage from each fireball infused GOW (although ML has weakness to fire so the damage normalizes) and I roll a 1 on the d6 for every roll therefore meaning 80 damage is the lowest damage I could possibly do.

Armor Class 17 (natural armor)
Hit Points 97 (13d8 + 39)

The average hp for ML is 97 so the success chance for this plan is pretty high if it all works properly? (And potentially there could be undead coming to attack the party while the Glyphs are being set up, creating some battle encounters rather than having a skip in time of like 3 days after starting the preparations.)

So this entire plan relies on what the conditions for a ML "activating" are and whether or not it would react to the familiar entering the lair. If it would say only come out of its sarcophagus once we opened it then that would make this plan far more viable theoretically.