First of all, you're OK to go through the tunnels whenever you want, and yes, Archibald will give you a blaster as soon as you enter Eeofol. Whether it's wise to do so will depend on how good your party is - the opposition in there is pretty tough. IIRC, you ought to be able to get to Lassiter's house easily enough. Piece of advice, though, save your game before you drink from the well!
This is the spoilerish bit, so I've concealed the text - highlight it if you want to read it. It deals with the need to go back and forth through the tunnels to Mount Thunderfist.
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There is a teleport between Eeofol and Harmondale which can only be opened from the Eeofol end. It's a hard place to reach until your party is well advanced because the countryside around it is stiff with dragons. It's near the coast in the extreme north, virtually in the centre between the east and west sides of the map. If your party is tough enough to take a few hits from the dragons, you might get it open, but don't take that as a guarantee. Good luck if you want to try it!
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