Please help me to be sure that such tactics are legal, by the rules as written. Thank you.

Spoilers following.

Doors fo Durin: I think it is possibile to use henarmath ability to watch the top encounter card, then choose to discard a card from hand with the matching first letter. 3 Victory Points achieved, quest easy finished. Is it legal?

Grasping Tentacle: "… attach this card to an attacking character as a Tentacle attachment with the text: "Attached character's Attackand Defenseare reduced to 0." I usually add a dummy attacker with 0 attack value and 0 defense like the Daughter of the Nimrodel, so I choose her to get the nasty attachment. Is it legal to attack with a bunch of chacters, one of which doesn't add any attack value?

And It seems that I can repeat this trick on the same character, adding more Tentacles attachment to the same characters, do you agree?

Thrashing Tentacle: "… deal the damage from the attack to 1 character an attacking player controls (ignoring defense)."

It says nothing about what happens (or NOT happens) to the Tentacle: did the damage afflict the Tentacle TOO or only one of my characters? In other words, is this like a sort of "reflection" effect or is it only a nasty side-effect?

In every case, can I use an effect like the one on Henarmath Riversong to watch the top encounter card just before choosing to attack it or not?

Doors fo Durin: I think it is possibile to use henarmath ability to watch the top encounter card, then choose to discard a card from hand with the matching first letter. 3 Victory Points achieved, quest easy finished. Is it legal?

Absolutely! You could even use Denethor's ability or the neutral event card with Gandalf's face that puts the top most card of the encounter discard pile back on top of the encounter deck.

Grasping Tentacle: - That is really the only safe way of attacking this tentacle. The more allies you have in play the better chance of beating the quest.

Thrashing Tentacle: - yeah this dude is bad. You don't do any damage to it. So having meat shields to redirect to is a key to surviving.

In every case, can I use an effect like the one on Henarmath Riversong to watch the top encounter card just before choosing to attack it or not?

Sure