The adventure definitely HINTS that it should be done this way. It says multiple times that the PCs will be in town searching for the location of Cragmaw Castle. When I read it the first time, it confused me...why would they be in town looking for the location of the castle when the goblins in the cave know where it is and they likely got that information from them? It wasn't until then that I reexamined the section on what the goblins know and I realized that it was meant to be vague on purpose. They could tell the PCs approximately where it was but they might wander a forest that size for days or even weeks without knowing its exact location.
Which is why the adventure seems to point to the logical conclusion that the PCs go to town to look for a guide who knows the exact location of the castle. Which no one does except for the Hobgoblins in the Redbrand Hideout or the Druid who is in in Thundertree. It's likely that the Hobgoblins die or at least refuse to tell the location. Which just means they'll skip Thundertree. The Goblin(Droop) who is with the Hobgoblins only remembers vague direction the same as the Goblins from the cave. Asking around town just gets you pointed toward the Druid by a couple of people.
So, the logical progression of the adventure is: Cave>Find out they need to go to the castle but only find out vague directions>Phandalin>Search for a guide who knows the area well enough to find the exact location of the castle>Get caught up with the Redbrands whether they want to or not as the Redbrands will attack them>Redbrand Hideout>Go on side quests for people in town who beg them to help OR Go to Thundertree to find the Druid who can guide them(or because one of the premade characters has a deathwish and wants to kill the dragon) OR Go to Cragmaw Castle (Any or all of the 3 in any order they want)>Wave Echo Cave
I don't have the DM booklet here before me but someone knows/suggests that capturing a Goblin patrol in the forest area might give further information about where the Castle is located.
(Gundren and/or Droop?)
Here's what I will do.
The Goblin in the Redbrand Hideout doesn't know where the castle is but tells them about the patrols.
If my party decides looking for one then I will use the Tracking rules in the DMG to see how fast they can locate one.
If they approach with speed they might catch the patrol before it reaches the castle at night.
A captured Goblin can give them the correct location.
If they approach with stealth they will not close the distance but find the castle without an encounter.
I'm just wondering if I should then add the patrol numbers to the ones already in the castle.
That might make it even more difficult.
Then again if my party is smart they will realize that patrols leave at dawn and return at night.
The Goblin could also tell them he heard that's how they do it.
Yes, I might play it like that