Otherwise, can someone please help with these questions:
1)How easy is it to travel between Returned Abeir and Faerun? How common?
2)Is there any trade between them?
3)Do most people even know of the other landmass's existence?
4)Did the authors intend "Returned Abeir" campaigns to be kept seperate from "Faerun" campaigns? I know DMs can do what they want, but I wonder why it was placed so far away?
Didn't parts of Abeir merge with parts of Toril (Faerun) - see page 87 of the FR campaign guide
"the two long-seperated siblings were joined anew, but only imperfectly. Portions of Abeir fell across Toril overlaying some lands and obliterating others..."
Returned Abeir is across the trackless sea, it crashed into Maztica and replaced the continent. Akanul and Tymanther were parts of Abeir
So to answer your question, teleport, hire a ship, go by road, or however else you travel in Faerun
The new continent of Returned Abeir is reachable through conventional travel as suggested by CeridLocke. Your best bet to reliable travel over the Trackless Sea and past Evermeet is getting a boat from Amn. This city state has invested the most resources into establishing trade with the oversea continents.
True Abeir, the out of phase dimensinal twin of Toril, can only be reached through planar travel. The best chance of crossing over is when the two dimensions of Abeir and Toril are alligned at a certain time every few decades or centuries. This is when the physical locations of the two planes are in phase with eachother and their planar boundaries overlap or lie close to eachother.
The joining of the out of phase dimensions could have looked like the image below if you where looking at it from a spelljamming vessel. Two dimension crashing into eachoter as their opposed circular motions are partially buffeted by eachothers features, therby leaving parts of the one on the other.
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[\sblock]1)How easy is it to travel between Returned Abeir and Faerun? How common?
I expect it's quite difficult. Voyage by sea is, of course, a straightforward possibility, but such a journey is sure to be dangerous, and there are bound to be many threats as one approaches the coast. Teleportation isn't a great option, considering how it generally relies on a clear mental picture of the intended spot. So, I'd say travel is neither simple nor common.
2)Is there any trade between them?
I expect Amn has some sort of trade agreement between the two, given its history of trying to shake down everyone for coin, even continents far to the west. I would say the Moonshaes might also have some form of trade, but considering the islands' conflict with Amn, there's a competition of interests, so if both were to trade with RA, I imagine they would do so with different nations. Beyond those two, I'd say Baldur's Gate might possibly have some form of trade, given its massive size and power, but it seems less likely (to me) than Amn's pact. How fruitful these trade agreements are is anyone's guess, especially considering how travel is dangerous.
3)Do most people even know of the other landmass's existence?
I don't know if I would say most do, but it certainly seems like knowledge of Abeir (both Returned and Lost) has spread. According to a brief mention in Wrath of the Blue Lady, books and tomes have been written about Laerakond and the return of Abeir, and have found their ways across the Realms. While at least some of these texts have been hoarded by monasteries and libraries, there must still be some circulating throughout Faerun. Given this, I figure that Candlekeep would, by now, have a section devoted to Laerakond.
4)Did the authors intend "Returned Abeir" campaigns to be kept seperate from "Faerun" campaigns? I know DMs can do what they want, but I wonder why it was placed so far away?
Given that people on Faerun have recognized RA in novels, I suspect that the two are meant to be recognized together, though not necessarily in every scenario. I suspect it was placed so far away because people who have a direct interest in RA can utilize it along with Faerun, and those who wish to keep it but leave it in the background can do so, and use the Trackless Sea as an effective buffer.