Yes, you can move the image out of the original range
"Within range of the illusion" refers to the 20 ft cube illusionary object or creature, NOT the 120 ft casting radius.
As long as you are within 120 ft radius of the illusionary object, you may move it to any other point within range. This lets illusionists keep one persistent Major Image for any amount of time if they can keep using actions to keep it near them.
What defines "any other point within range" however, is up to discussion. It could mean you may move the object either 120 ft within the caster, or 120 ft within the illusionary object.
Let's consider this from two perspectives: RAW and RAF.
RAW: The Mirror Image duplicates are only around the original target of Mirror Image (Illusory you, or real you)
The only official guidance we have here may be seen to confuse more than clarify. It is found in a tweet from Jeremy Crawford:
Question: If you have mirror images out and then cast invisibility on yourself, do the mirror images also become invisible?
Jeremy Crawford: Mirror image creates illusory duplicates of you that imitate your appearance. If your appearance goes away—you become invisible, for example—then the duplicates imitate that too.
This seems to give us clear guidance at first: a spell cast upon you also seems to apply to your illusory duplicates. But note that the spell in question (Invisibility) altered your appearance: but the illusory duplicates from Mirror Image do not change your appearance, but rather create the appearance of other things around you.
Invoke Duplicity creates a "perfect illusion of yourself" (PHB, p. 63). It does not necessarily create an illusion of the effect of spells that are near you. As an example, if you had the spell Minor Illusion to create the illusion of a 5' cube of granite in your own space, then used the Invoke Duplicity ability (or did these two things in the opposite order), your Duplicity illusion would not also have a block of granite surrounding it, because the granite is not part of "yourself." Similarly, the Mirror Image duplicates flitting around you are not "yourself," (although they look like you), and are thus not automatically recreated by the Invoke Duplicity illusion.
However, you could certainly cast Mirror Image through the illusory double (Case 2), and give it (but not you) several Mirror Image duplicates, since, as you mentioned:
you can cast Spells as though you were in the illusion’s space (PHB, Trickery Domain)
and
Three illusory duplicates of yourself [from Mirror Image] appear in your space. (PHB, p. 260)
Even though Mirror Image has a range of "self," the feature that permits you to cast Spells as though you were in the illusion's space is designed to expand the range of your spells so that it appears that the illusion is the one casting them. For example, a multiclass character could cast Thunderwave (range of "Self (15 foot cube)") centered on the Invoke Duplicity illusory version of themselves. Also, note that it has been established that you could cast spells with the range of touch against targets that are adjacent to your Invoke Duplicity illusion. Similarly, you could cast Mirror Image centered upon your Invoke Duplicity duplicate, rather than upon yourself.
RAW, it's debatable whether they would then proceed to mimic your actions or the actions of the illusory duplicate, but regardless, they would certainly appear in its space.
RAF: Sure, duplicate the Mirror Image doubles too. Who cares? And it's cool!
The RAW reading above is reasonable, but inherently problematic. If the Mirror Image duplicates don't count as part of your appearance, do your clothes? Does your Light cantrip cast upon your shield? What exactly does count as "you" for the purposes of this illusion? The argument leads us down a slippery slope.
Also, giving the Invoke Duplicity illusory version of you access to the Mirror Image duplicates that flit around the real you for free when you cast Mirror Image will have no mechanical effect on the game. Note that, unlike the Mirror Image duplicates of yourself, your Invoke Duplicity illusion does not disappear if it is struck, but only when 1 minute passes (or you drop concentration), so there's no benefit to making it harder to hit. Also, unlike the Silent Image illusion spell, it does not become faint or ineffective if someone realizes that it is just an illusion. It will still permit you to cast spells as though you were in its space and give you advantage on attacks against creatures that both you and it are within 5' of even if everyone in combat realizes which of you is real and which is an illusion. The mechanical effects of the Invoke Duplicity Illusion do not at all depend on the opponent knowing (or not knowing) which one of you is real.
So giving the Invoke Duplicity illusion three more illusory versions of itself (that remain in its space) whenever you cast Mirror Image on yourself (or if you already cast Mirror Image before creating the Invoke Duplicity illusion) would have little to no mechanical effect on the game, other than to make the combat more chaotic. This is especially noteworthy because this is an effect that a player could already create RAW by casting Mirror Image twice, once on themselves and once on their Invoke Duplicity illusion (since it is not concentration, and will validly apply to two different targets without invoking the "combining magical effects" rule). Requiring a player to cast Mirror Image twice seems like an unreasonable waste of resources for something with no mechanical benefit, so as a DM I would permit allow the Invoke Duplicity illusion to have the Mirror Image duplicates around itself in both Case 1 and Case 3.
Best Answer
Major image only lets you create one image of yourself
Note the singular there. Only one illusion of one creature can be created by the spell.
No, it doesn't. Note that it says "other" visible phenomenon. That means visible phenomenon other than the two categories (object and creature) previously talked about. It doesn't mean "other phenomena" in the sense that you can ignore the limitations previously mentioned in the spell. The phrase is only there to allow you to create non-creature, non-object illusions (eg a rainbow or fog).
If major image was designed to allow this they would not have put the words "a creature" in the first line of the spell. Instead they would have said "objects or creatures or other phenomena". But they didn't and thus the RAW of this spell simply does not allow it to be used in the way that you want it to.
There is already a spell that does exactly what you want to do: mirror image
Mirror image says:
This is exactly what you are trying to do with major image and since it won't work with major image you should just use it instead. The spell is literally tailor-made to achieve the effect you wanted. Mirror image is even a lower spell level than major image so you can get the same effect by using less resources.
Major image and mirror image are different spells that are designed to do very different things
Mirror image is a one trick pony; all it can do is create illusory duplicates of yourself to aid you in battle. Useful if you want that one particular thing, but not so useful otherwise. Major image is a very versatile spell that can be used for all manner of strategies both in battle and out. It can create illusory hiding places. It can create illusions of threats to scare off enemies. You can create illusion of treasure to set up a trap. You can create a single illusion of yourself to talk to the enemy leader and avoid exposing yourself to danger.
They are simply two spells with two different effects and ways they were designed to be used.
Regardless, the wording of major image explicitly shuts down what you are trying to use it for, it simply was not designed to duplicate the effects of mirror image.
Even if major image could duplicate mirror image's effects (again, it absolutely cannot), it would be strictly worse than mirror image in just about every way in doing the one thing mirror image was designed to do.
1. Major image uses a higher spell slot
2. Major image requires concentration (mirror image does not)
3. Major image requires you to use your action to make the images move with you (mirror image does it for free)
4. Major image disappears upon simple physical examination or an ability check. Mirror image requires each image to be hit with an attack.
5. Major image’s illusions disappear at the same time. Mirror image only has the hit one disappear.
There is simply no conceivable way that using major image the way you imagine it would be better than using mirror image instead.