I'm not sure it was necessary to start labelling me with insulting terms. I responded to your original post in a respectful way. (Though from your use of each term, I'm guessing you have different definitions of each, though you clearly understood they were derogatory)
First off, I apologize if I offended you. You seem to think your box example is perfectly reasonable. I think your box example is beyond absurd and this might be a difference between how we view illusions.
Although the game is often played with tiny miniatures on a table and possibly a grid, there are a whole lot of things going on behind the scenes. The Halfling is not just standing totally motionless while fighting, he is jumping to the side, ducking under swings, swinging low with his weapon some times, stabbing high with it other times, etc., etc. etc. in my imagination.
Now your example comes along and you put that Halfing in a box. Imagine this from the Halfling's POV and not the rules perspective. He has a hole he can see out of and the main light appears to be coming from that hole (because after all, an illusion of a box would look like a box on the inside as well). Most of his view of the foe is denied unless he moves right up next to that hole and peers out. The same applies to the hole the wizard created to have him attack out of. It restricts his attack. He cannot do an overhead attack through that hole, it will probably have to be a predictable thrusting motion.
From your POV (which appears to be rules based), the halfling gets advantage on his attacks.
From my POV (which is imagination based), the halfling gets disadvantage because he is fighting from a box with two holes it in to see and attack through. Minimally the foe has 3/4ths cover. The wizard practically tied the halfling up.
My intent was not to insult you, but to illustrate that not all DMs will view this cool tactic of yours as cool (and some DMs might view it as rule lawyer-like) . And the reason might be because how the imagination works with regard to illusions in the game. Illusions can be distracting, but they should often be similarly perceivable to every creature on the battleground.
So yes, I consider your box example to be very unreasonable as a way to both protect the Halfling and to give him the huge advantage bonus with a simple cantrip. But from what you yourself has said in your podcast and in many of your class optimization pages, you prefer wizard capabilities that are almost 100% guaranteed as opposed to ones where there is a fair chance of failure. And, I'm ok with that. I wouldn't recommend it like you do as a type of bread and butter for wizard players as per your dislike of Shocking Grasp, but to each their own.
Instead, an example of protecting the Halfling with Minor Illusion that I might give is to create a cylindrical wall (with a roof or lid) around the halfling that only goes around 75% of the way. From the foe's POV, there is now a closed cylinder around the Hafling. The foe cannot see through it to know that there is an exit out the back. The Halfling is protected. The foe will probably attack the Barbarian. Now the foe might be curious and walk around the Barbarian (without provoking in 5E) if terrain and situation permits and find out that the Hafling is not really protected at all, but that's a DM call on monster action based on what the monster knows. Alternatively, it could be a box with the back side missing instead of a partial cylinder.
With this type of solution, the Halfling is protected until the Halfing walks out the back of the illusory partial cylinder, comes around and re-attacks the foe. This type of solution doesn't try to backhandedly shoehorn the "opponent cannot see me" advantage rules into the situation and attempting to use a simple cantrip to partially cast the equivalent of Invisibility on the Halfling. It basically does what you stated was your goal, it attempts to protect the Halfling. It's not 100% like you prefer, but it doesn't hand out free advantage either. And this solution is initiative dependent. If the foe attacks before the Halfling, it does its job. If not, the player of the Halfling has to decide whether he wants to maintain the protection or not.