Character's name emphasized twice, character description, and "specific locations"

Welcome to the board. I'm not a screenwriting guru nor do I play one on TV, so these are just my thoughts.

They are all together, sitting around the big table, when a POSTMAN steps into the house.
...
The POSTMAN, (mid 30s), short, thin, bearded, stands in front of him, and extends his hand to shake Papa Simpson's hand.

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That seems kinda unusual. And the description gets in the way of the action. I'd do it the standard (or at least what I think may be the standard) way, describe upon intro, e.g.

They are all together, sitting around the big table, when a POSTMAN (mid 30s, short, thin, bearded) steps into the house.
...
The Postman stands in front of him and extends his hand to shake Papa Simpson's hand.

...That's assuming this description is somehow vital. Do you really need to narrow things down so much? What if they cast an older, heavier guy without a beard? Does this matter, does it affect the story in any way? If it doesn't then just call him a POSTMAN and forget the trivial details. From the label, we'll assume he's a uniformed postman.

2, Is it fine to give a brief character description of someone if s/he only plays an important role in one or two scenes, but then will never ever appear again?

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If the description is needed, for story purposes, to identify this character in some specific way that's significant, then it's needed. Sometimes just a name or occupation label will do just fine.

3, Is it fine to write this in the Slugline: EXT. CHOCOLATE FACTORY - NEAR THE MAIN BUILDING - DAY so if the location is big, but some scenes take place closer to the building, can I use near, further, next to etc... to specify the actual place?

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That seems okay to me.

EXT. CHOCOLATE FACTORY - NEAR THE MAIN BUILDING - DAY

EXT. CHOCOLATE FACTORY - LOADING BAY DOOR - DAY

EXT. CHOCOLATE FACTORY - MAIN ENTRANCE - DAY

etc. are all different locations.

-Derek

I'm even less a screenwriting guru, but my understanding is that unless the physical appearance of a character plays a part in the plot, less description is always ideal. Do we need to know you envision him as a man of slight build with a beard? (Which is fine, of course. I usually cast working actors in my mental movie of the novels I write. Imagine him however you like.) What if he's tall and thin, or has a shaved head? Does that affect how the scenes in which he appears work?

Often just the decade of a character's age is sufficient for casting.