1) I don't know; I have never tried that.

2) Yes, it is gone for all purposes.

Yea i often destroy them if i have multiple..... but never the ones with useful adjacency bonuses, the one that has utterly useless adjacency are the military adjacency.... so useless it makes my eye twitch whenever i see one! lol

Trade resource buffs stack. Most of them are better than anything you can replace them with.

1. I do it when I'm specializing a planet. No problems there. 

Seriously??.... all this time i have been making sure i have at least one of everything and trading the excess away because i thought its not doing anything for me - all because of what that description said.

The evidence is in things like Autocarpus Viriles which affect growth on all colonies.  Hover over the growth display on the planet screen and you will see the bonus from Autocarpus Viriles and you will see it change with multiple copies.  The same is true for Snuggler Colonies, Monsatium Deposits, and probably others I can't think of at the moment.

By the end of the game, I will own all of the Autocarpus Viriles in the Galaxy, either by purchase or conquest, but that is only one of my obsessions.

My obsession is to reach double constructors on a medium hull and one constructor on a tiny hull so i can make use of all those useless left over scouts! lol

A constructor on a tiny hull used to be easy, but things changed! lol

yeah, I used to destroy them but noticed they seemed to stack - nice for confirmation.

No, its the one in, you can build only one per player.

(thought no one can build it^^)

Tbh I have always wondered why they are using Trade Resources as wonders since they are unbuildable.

Possibly "one per race" could mean that if you have five races in the game, (but probably have to include minors too) it will spawn five of each trade resource on the map?

I highly doubt that is true, but if it was, then technically the description is 100% correct even though it is very deceptive.

Military adjacency is actually really useful if you get it on one of your major military production worlds. Readiness Center +2, the trade resource +2, maybe another military building, and you're sitting at +5 for one of those unique military buildings (Hyperion Logistics, etc). Requires some planning ahead with terraforming, but it can pay off big. That said, its obviously situational.