Smog
If you are playing longer sessions, the Loremaster should be breaking them up and refreshing your pool halfway through. The normal expectation for a "session" of TOR is about 4 hours. If you're playing all-day marathons for 8-10 hours, you should be refreshing the Fellowship Pool when you break for dinner or whatever the approximate halfway point is."
I had a look at both core rules book and Darkening of Mirkwood book and it never says that a session last about 4 hours. It only says that "usually, a full adventure should not take more than two or three sessions of play to complete" ==> no indication of session duration (unless I missed it in the book). And having the pool refreshed after 2 hours (a small evening game), 4 hours (a good evening / afternoon game) or 20 hours (a full week-end game) isn't the same : it's not the same game.
Now that's right that it says that the "A company’s Fellowship pool is completely refreshed at the beginning of each new gaming session.". If at the end of a session, the characters are resting in a relatively safe place where they can have a break, then I'm fine with that "refresh fellowship pool". But how do you explain having that "refresh fellowship pool" when the session ends with characters in the darkest place of the Wild Lands (Mirkwood/Dol Guldur), with no secure shelter, with adversaries all around them ? It is a completely artificial mechanism, unrelated to the real context of the characters.
I personnaly hope that this "session / fellowship pool" is removed and replaced by something less artificial in TOR 2nd edition (maybe there is already something about that in the changes that we know about 2nd ed., I didn't check).
Smog
I'm also unsure why you seem so surprised by a PC falling to the Shadow in this adventure. I absolutely get what you're saying about it feeling like an inevitable spiral, but they are in the deadliest and darkest location in all of the base region (Wilderland/Rhovanion). If a player doesn't run a very serious and real risk of losing their character here, then the game isn't being terribly realistic or immersive. I know if I were a player at the table and I didn't feel seriously threatened by the prospect of going to Dol Guldur, I'd question the faithfulness of the system to the source material.
I'm not surprised that a PC is failing due to the Shadow, this is in the heart of that game and of Middle Earth and I expect that in a Darkening of Mirkwood campaign, several players would have to play several characters after having some dying, retiring or failing to the Shadow.
What I'm surprised is that we can loose a character after having 5 bouts of madness in a very very short time due to that "miserable state spiral" (that we, in french TOR community, consider as a system bug).
Smog
With all due respect, this is completely wrong and does not adhere to the source material or one of Tolkien's greatest recurring themes: that Shadow is fleeting and that hope and light can be found in the darkest of places. Here is a direct excerpt that completely contradicts what you've said above:
...
Players should always be presented with the chance to regain hope. It may be less common in places of shadow, but it should always be a possibility. It is the Loremaster's job to mediate the systems and adapt as necessary in order to present the players with a challenging but fun experience. If they are perhaps too inexperienced for where they are and getting drowned in shadow, and you think it's too much, then perhaps the full moon should appear just in time and in all of his glory to the dismay of the enemies, or perhaps a cool breeze with the fresh smell of a far green country suddenly blows in from the west and revitalizes your heroes.
Carcharoth
I see what you're saying, but I do believe that point #3 is debatable. While I agree that Dol Guldur is and should be treated as a dark and miserable place, if going there is causing your party to plummet into Shadow, then I don't see why you can't invent situations that might help inspire hope, even if just for the sake of having a fun game. Why not have the players stumble upon a fragment of ruins leftover from the kingdom of Oropher? Why not have a small beam of wholesome sunlight cut through a break in the forest canopy to remind the party of the simple joys in life? If the players at least have a shot at sustaining themselves spiritually (regaining Hope), then it probably wouldn't feel so bad to lose characters to Shadow, and could make for some memorable story moments.
Thank you both Smog and Carcharoth for that remind, you are right. This is a theme point of Middle Earth that I underuse and I will correct my own game

I would suggest that in the 2nd edition, this advice is given to the Loremaster in the book. I've checked v1 rules and DOM books (at least until 2950) and i've found nothing of that inside and thus players playing strictly by the "books rules / scenario descriptions" won't use those hints of hope moments.
Now for the "spiral" issue, probably that it would have given more time to the characters but imo getting 1 more hope point wouldn't have delayed long the miserable status and bout of madness.