So, you'll need the mists to get them to Souragne (Night of the Walking Dead), Har Akir (Touch of Death) , and back to the core for Feast of Goblyns, huh? 'Ship of Horror' could be the party's transport in the direction of Darkon for From the Shadows. In the core, travel is less troublesome, unless one of the Dark Lords closes the domain borders. Kind of thinking out loud, I'd need to read the modules again. It has been a while. 'Teleportation' magic might send the PCs to a 'random' location in the demiplane. The mistways might be an option via the Vistani, I can't remember the particulars of that though.
Nightmare Lands was part of the core until the conjunction, after that it's one of the Islands of Terror I believe.
Use the land itself to pushed them onwards by giving subtle hints of how unreal the place seems. Make the air threatening, anyone who does a detect evil should get a response of everything being out against you. When in doubt bring in the gypsies and do a fortune telling, referencing how they are outsiders. By dropping hints of an escape, by npc or whatever, it might get them to ask the right questions. Let it sink in slowly, when they fear the mention of mist as a player, you got it right.
Edit, I realized you were asking a literal question of moving forward. There are some mistways that are "stable" but the vistani are the only ones who can navigate the mists accurately. While the domains change all the time, the core stays relatively the same until the grand conjunction. But the Demiplane of Dreads book should cover that.
Ok, in the end I decided that Dulcimae (from "Night of the Walking Dead") and Valana (from "Touch of Death") with the respective families, will be instead the same people. There will be only Dulcimae and her family (her mother is not present becouse died some time before, Dulcimae must be the female leader of her family, just the same way as Valana is), and they will play the role of Valana's family in "Touch of Death".
This way I enstablished a strong link between the two adventures.
But I can not use this the vistani over and over to push the party forward every time, and I still need to make the PCs go from Har'Akir to Kartakas, so . . .
. . . the PCs exit from the mausoleum at the end of ToD, and they are surrounded by a great sandstorm, and a number of "shapes" can be seen, they are not discernible in the sandstorm, but they are approaching...slowly. The desert zombies grab them and drag them in the mist, out of Har'Akir to Kartakas. This was the last order given to the zombies by their lord.
While the PCs are in Kartakas I will throw in the first 2 adventures found in the third book of "The Nightmare Lands" boxed set.
The first of the two is nothing more of a dream (that will occurr during the PCs permanency in the prison of Harmonia) followed by a later encounter (occurring at night in a non specific alley).
The second will take place along the road from Harmonia to Skald (it needs only a village, and I can place it almost everywhere).
These two adventures should lead the PCs (hopefully after they completed Feast of Goblyns) to Nova Vaasa in search of dr. Ilhausen, where they will play the third chapter of the "Nightmare Lands" boxed set adventures (in Egertus), and finally set sails toward the Nightmare Lands (in the fourth and last chapter). This last adventure ends with the PCs coming back to dr. Illhousen's clinic in Egertus (Nova Vaasa). From Nova Vaasa, shouldn't be too hard to make them embark on "The Ship of Horror" (but I have not read it yet at all). . .
. . .OR. . .
The ship of horror could be the same ship they are supposed to use to go to the Nightmare Lands, so I would avoid to use the same idea (the travel by ship) twice in a row (better).
P.S.: the "Ravenloft 25th anniversary" module and "Bleak House, the Death of Rudolph Van Richten", are on their way to my house, and I would like to integrate them. . .so this post will see moooooore comments, sorry to bother you all :-)
P.P.S.: do you suggest to integrate "Castle Ravenloft" before "From the Shadows" + "Roots of Evil" (so the PCs will know what they are facing in the latter two modules, and they will be surprised to find again the evil vampire 'alive'), or using it in the end, after the GC is over (so the PCs will have the chance to wipe one and for all the evil of Strahd)?
Order of modules:
1 - Night of the Walking Dead
2 - Touch of Death
3 - Feast of Goblyns + Nightmare Lands 1&2
4 - Nightmare Lands 3
5- Ship of Horrors
6 - Nightmare Lands 4
7 - From the Shadows
8 - Roots of Evil
9 - Castle Ravenloft (maybe)
I really don't know how to insert Bleak House boxed set (it starts with another shipwrek, and I don't like to use the same expedient more than once)
Yes, resorting to the mists, again, is not very satisfying. Players with any knowledge of Ravenloft will likely understand your position though. With regard to Bleak House, it occured after The Grand Conjunction, around the time of Grim Harvest. The accessory Chilling Tales has a bunch of adventures which include Van Ritchen. It might be some additional help if you want to introduce the party to Van Richten beforehand. I've dug out the old modules and I'm giving them a more thorough look right now.
I will check "Chilling Tales" for sure then, the death of Van Richten will have a deeper impact if the PCs met him before.
By the way, I don't care for official timeline, as you said, the Nightmare Lands should be part of the Core before the GC is complete, but they will be already an isle in my campaign.
I don't care for Van Richten death date either 
About the grim harvest, I read some bad rewiev on the fraternityofshadows website, it seems like the rules to create undead PCs are somewhat of a mess, and given the price of any RL book on the Bay, I kept myself away from it 'till now.
Someone suggested me to make the ghost of Dulcimae/Valana a recurring NPC, appearing and reciting couple of the exad everytime one is fulfilled. . . I like this idea so much!