can legally show video boxes or posters for real movies, either up close or in the background.
Well... no.
This goes beyond video boxes and posters on a wall. It covers things like, let's say, billboards in the distance that may be showing a product.
For instance... let's say you have a bloody knife-stabbing and killing with a Coca-Cola billboard visible in the background. Chances are (with today's legal weasels) that Coca-Cola will not appreciate having their product associated with a violent crime.
In reality, that crime (or whatever other scenario you may have) is not really related to the background or ambience. The products themselves may feel differently, though.
That is why insurance guards such as "E&O" (Errors and Ommisions) exist.
There are a few companies that make "generic" things. They can provide generic beer bottles, soda cans, posters, and many other background/ambience type things. They'd likley have posters and empty video cases.
If you really wanted to get some networking points (while covering your bases)... since you're making an indie-film, why not contact other indie-film producers and ask for casings & posters? You could easily get (with total permissions) a tonne of posters and inserts to decorate your video store with. It would look like a proper store, and the people you meet & get materials from would be appreciating the exposure.
You win, they win.
legally mention any movie I want without showing the box
No idea. Interesting question.