Perhaps WotC will clear this up for one day
If we take a look at the inconsistency -- page 83:
PARTY SIZE
The preceding guidelines assume that you have a party consisting of three to five adventurers.
If the party contains fewer than three characters, apply the next highest multiplier on the Encounter Multipliers table. For example, apply a multiplier of 1.5 when the characters fight a single monster, and a multiplier of 5 for groups of fifteen or more monsters.
If the party contains six or more characters, use the next lowest multiplier on the table. Use a multiplier of 0.5 for a single monster.
and the recommendations of the modules I think publication date matters. 5th edition was waaay more popular than anyone guessed it would be and I think 3 - 5 number was based on the assumption that it would be hard to get 7 people together on the weekend.
Again with Hordes of the Underdark there is no party size mentioned but the module gives you some pretty powerful NPCs / Followers including a myconid and a hook horror. So as the game has progressed the party sizes have been increasing. So I don't think we can say 3-5 is the recommendation. Neither can we say that 6 is the recommendation. 6 is the right number for other reasons -- a. six more fun b. this is BG and not just DnD
Anyway, I didn't understand every point the OP was making but agreement on 1-5, 9 and 17.
Edit: I think the DM guide is really just saying "we're assuming this number but if you want a different number here are some suggestions". Which makes it something other than a recommendation -- the recommendations come in the modules.
