Will do my best to answer these with the current knowledge I have at hand about trading:
This has been discussed heavily, but we still have no final plans for restrictions on trading. Trading recently has gone on the back burner for a bit while we focus on getting mainly CTC and lobbies out ASAP (They come as a pair, we cannot have one without the other). Before this hold on tradings development, the core functionality of being able to trade any item in your inventory (i.e clothes and weapons) was being implemented and from what I understood almost finished. Of course from there, things like gold and mats (if decided to be tradable) will be added as options, and once the core of trading has been tested, from there restrictions on item rarity (if any) would be implemented.
Yes, a server side check will be fundamental to trading so no client side changes can change the outcome of a agreed upon trade. From what I understand, (forgive me as trading hasnt been discussed in about a month, per my previous statement about focusing on our next expansion) this is already being developed as a foundation of trading.
Currently no, but it is an interesting idea. Right now our idea is trading will be available to any pirate, as it will be on every single player card, giving the opportunity for any pirate to initate or opt out of trading.
As of right now trading would be stand alone. It is a relatively big addition to the game that likely warrants it's own update, but of course if trading coincidences with a future update that will be finished around its internal ETA (whenever that is determined by the development team), it is possible there could be some tie in to that, but as of right now no.
This idea has been looked at but nothing for it has started development or made it past the initial very basic "discussion" phase.