My sim met a girl from China and they became friends (I remember she was a book merchant), He invited her home and they were able to marry with no problems and she moved into his home in Sunlit Tides. I decided to move the family to Twinbrook and used the Traveler method following the directions on this website. The whole family moved with no issues and I did notice that they didn't lose relationships with the sims from Sunlit Tides which I figure is a feature of the Traveler mod. I decide to take a vacation to China once again but now when my sim arrives ALL of the China residents are bugged. There are no merchants at the registers and I can't even click on the resident's homes. The game still thinks that the residents are there, but none of them are selectable/visible. I've tried resetting all sims in China but that didn't work. Is it possible that by using the Traveler mod to move to another town permanently, I've messed up the WA vacation worlds? I haven't tried visiting France and Egypt again. Or could it be that because his wife was a book merchant and he moved her in that messed things up?

Some ideas off the top of my head:
I know its an issue if you travel from A to B to C without going back to A first. Did you do that by accident?

Did you try the "fix homeworld" command?
chatterbox/topic3652

Switching hometowns with Traveler shouldn't have borked anything in the vacation worlds. However, it can cause problems if you invite a foreign sim and then have them move in while they're visiting. Tourists are mini-sims, without all their data, and can cause all kinds of issues.

For the mising merchants, try installing Register. But don't run a Fix Homeworld in China: this command should only ever be run from your sims' homeworld, whatever it happens to be.

For the missing residents, have you tried evicting them in Edit Town and placing them back in their homes? It would be tedious, so you could try with one or two households and quit without saving if it didn't help.

@KevinL5275 Well technically yes I did travel that way, but I didn't realize it would be an issue to travel to China from Twinbrook because I'd made Twinbrook my new permanent homeworld when I moved. I haven't tried the "fix homeworld" command but I will try and see what happens.

@puzzlezaddict Yes I did move her in while she was visiting but I didn't know that would cause bugs, in fact I thought that was the proper way to move in foreign sims...oops
I do have Register installed and when I travel to China I'm bombarded with script errors from register. I thought it was trying to assign merchants for the registers but all the residents are borked so its not working properly. When I go to edit town and try to evict the residents it just causes UI problems and I can't click on anything anymore and have to crash to desktop.

Ok I think I've finally fixed the issue, what I did was go to every occupied home and reset the lot using MasterController. Resetting every home lot unborked the residents and they were now able to be selected and are no longer invisible. I also used Master Controller and Debug Enabler to make sure every WA register had a sim tending it. I used the Register mod to select a special merchant and now everything seems to be working as it should. Thanks for the help, I'm off to go explore some tombs!