Sorry for the minor necro, but I just came across this. I had originally planned to do the same, at least with prologues for levels 3-4, to get the party (and players, let's be honest) invested emotionally in the vale before it burns to the ground. In the end people wanted a shorter campaign so we just dove straight in with the ambush. Below are some thoughts from that prep, but definitely keen to hear more from others!
One idea I had was to use
The Scroll Thief from the Tyranny of Dragons AL season 1, set it in Brindol and change the 3 stolen books to things that tie into the RHOD story (eg the book stolen from the wizard could be a book on Tiamat stolen from Immerstal; another could point to Rhest, and the lienage one could maybe connect to the Vraath Keep adventure hook in RHOD), and have Miha the spy behind it all. If run at 1st-3rd level, the payoff will be quite a fair bit later, and hopefully pretty cool.
Another AL option is
DDEX2-10 Cloaks and Shadows (from the Elemental Evil season, with good reviews), reworking the elemental cult into Kul's people. Perhaps Miha has set up the cult cell (having seduced Zor) and now that the invasion is imminent, is removing any potential telltales. This would be set in Brindol (or even Dennovar), and has a political intrigue element in which the party are actually hiding form the law, having been framed for a murder.
There are probably tons of other AL adventures that would fit in pretty well too.
Wolves of Welton can be placed almost anywhere in the vale, and is a highly rated (and free) 2nd-3rd level adventure.
WOTC actually published a short
prologue adventure for level 4. It's pretty lame IMHO, but could be built on. Weirdly, they don't even specify a potential location in the vale, other than that it's "in a hillside cave at the fringes of populated land (near where the PCs are adventuring), living off of pigs and sheep stolen from nearby farmsteads," which fits pretty much every single named locaton in RHOD.
I tried to find a perfect adventure for Baron Trask in Elsircross because the lovely little detail in RHOD is screaming for a prologue or side quest adventure, but no luck so far. Ideally it would be something emphasising his evil alignment, and giving the party either a reason to hate him or overthrow him (but ultimately to point out that good and evil are not static racial features).
I really like your idea of reskinning some of the ROTFM ten towns early quests, but I haven't played through any of them to know which ones are the most fun.
From the Essentials Kit, my personal favourites were Gnomengarde, a much-revamped Dwaren Excavation (replacing the TPK gem trap with a more Indiana Jones-eque slowly collapsing ceiling, and spending time to describe the temple's details), and Axeholm (again, building up the tragic story for the party to learn - this one could give the party a boon when negotiating with the dwarves in RHOD perhaps; would need debuffing to level 3-4).
From LMOP I would think either of the rescue missions (Cragmaw Hideout and Cragmaw Castle) could be worked in fairly easily, with the castle being in the Witchwood near any of Drellin's Ferry, Talar, or Elsircross. As could Thundertree, but you'd have to be confident you can get
Venomfang Ozzyrandion out of there alive.