This thread pops up a remarkable number of times. I had a somewhat similar issue in Book 5 ("How do we get into the Runeforge again?") and I didn't hesitate to suggest at the beginning of the next session, "Why don't you go through your notes again before we begin?"
If they can take their eyes off the naked succubus for a moment, they will notice the phrases:
"I can't wait until the real raid. This down deserves a burning, that's for sure."
"...once Malfeshnekor's released and under her command, we won't need to worry about being subtle..."
"Burning her father's remains at the Thistletop shrine seems to have started the transformation..."
"Hopefully when she offers Sandpoint to Lamashtu's fires..."
In short, Thistletop is mentioned twice, burning Sandpoint is mentioned twice, and it sounds like they're well on their way to releasing Malfeshnekor, whoever that is.
As many GMs have posted on many other threads along these lines, if the party can't figure that heap-o-clues out and doesn't care to do any legwork to find goblin scouting parties preparing for the "big assault", then it's time to map out the sacking of Sandpoint.
Honestly, if they can't figure out the note and they don't leave town, there's not much you can do for them, but if you're *REALLY* nice, have Shalelu show up and tell them something's wrong at Thistletop. Really heavy-handed, but some groups need a clonk with the clue hammer.
Oh, absolutely! Having them sitting around on their heinies and then Shalelu shows up to talk about increased goblin activity is perfect!
They'll say, "Hey... didn't we hear the name 'Thistletop' before?"
Great plan!
I think that, technically, the pc's are "supposed" to have received a goblin briefing from Shalelu, including descriptions of the Thistletop leader Ripnugget and the bugbear Bruthazmus prior to Ameiko being kidnapped. Sequence then is: pc's learn about powerful local goblin tribe, Tsuto's journal references the tribe in some detail and that the tribe will be part of another raid - Hey, maybe we should check it out!
Reversing the order gets to the same place, except you lose an advantage of the "scripted" order. Shalelu's briefing in conjunction with the original attack justifies Hemlock leaving for Magnimar and Shalelu leaving to find out more about the goblins - destination deliberately ambiguous. (This in turn explains why Bethana brings Tsuto's note to the pc's - she would have almost certainly taken it to Hemlock or Shalelu first if they had been in town.) If Hemlock and/or Shalelu haven't left you now have to explain why they don't go to Thistletop themselves. Hemlock can perhaps believe sending the pc's as scouts is a good idea since it puts none of his men at risk. But it's much harder to explain why Shalelu doesn't tag along. If you want her to tag along, great but she's 3-4 levels above the pc's and either will need to be underplayed or runs the risk of outshining the pc's (never a good thing for an NPC.)
Have your pc's been sharing information with anyone in town? The Mayor? Hemlock? They could easily suggest 'someone' should go see what those crazy Thistletop goblins are up to. Of course, if you intend to include Shalelu in Book 3 (Hook Mountain) you're best served to introduce her now... Perhaps she "sends" the pc's to Thistletop while she checks out the Mosswood goblins - the larger tribe and greater threat to Sandpoint. No one knows Bruthazmus has thrown in with Nualia or that she has yeth hounds or that she has additional mercenaries, etc.