A very good walkthrough for Pokemon diamond is the website Psypoke.It also shows many tips and lists of berries and where to get them,Tm's and HM's, and all sorts of stuff for Pokemon diamond and pearl.It also shows the same things for emerald and all the Pokemon games so far, plus they are making a guide for the new Pokemon platinum.It also has the best pokedex on the internet(in my opinion) and shows locations on click on maps where you can click on the route and figure out what Pokemon are there and how to get them(pokeradar,only in diamond/pearl, etc.)The website address is posted below as a related link.
There Is A Pokemon Diamond Walkthrough If U Go To Pokemon .Marriland.com
You got to go to iron island and at the end of it you find it
Marriland is youtube walkthrough maker he has done a Diamond/Pearl walkthrough a japenese platinum walkthrough and a English one
this is a walkthrough to help anyone who needs to get there http://pokedream.com/games/diamondpearl/walkthrough/
just search google for Pokemon diamond walkthrough
well if you want to find that up look up STEALING THE DIAMOND WALKTHROUGH
at the peak or top of mt.coronet there is a walkthrough on youtube
Serebii.net is usually a good place to start but for an actual walkthrough just Google Pokemon diamond and pearl walkthrough and you will get multiple results
To get a swampert just choose Muskie as your partner and journey with it untill it evolves. If you cannot choose Mudkip, look near lakes and rivers maby.
For Platinum i know an extremely helpful walkthrough website called pokemonplatinum.net. it shows you what to do next if your stuck and i used it multiple times in sticky situations. I'm not sure of a website for diamond and pearl sorry
ABXY
Sadly, no. The creators of the game forgot to add in the Pokedex, so trading is impossible without GameShark. :(