Council of Elrond

Most (all) Ent allies have text indicating they may not be "Attacked" by automatic attacks and attacks keyed to sites. (They look like trees when they aren't moving.)

This isn't the same thing as "may not be assigned a strike" correct? They can be a target of a strike if so chosen by the player whose company is under attack, correct?

No. They may not be the target of strikes from any such attacks, and may not be assigned such.

The only alternative interpretation is that such an ally prevents hazard creatures from being played keyed to his site at all and that a company with such an ally can never enter a site with an automatic attack.

I don't see that as being the only "alternative" interpretation, and because you of all people refuse to acknowledge 'interpretation' as having anything to do with rules, I'm surprised there isn't a ruling on this issue. I'm not aware of anything in the rules allowing for a character to be attacked, only a company. It is the card text of Ent Allies and Slayers that ambiguizes (which is not a word) the issue.

In other words characters aren't attacked, companies are, so the wording is already bad, (broken), and unless ICE ever published a clarification, I gather they have not, and obviously will not, I see no alternatve but to form a ruling among the players.

I do not see any sense in abandoning the "spirit of the game" rule of thumb, and as such I feel your 'interpretation' to be less sensible than mine.

I have no qualms about agreeing to disagree, and unless someone can direct this topic to an official ruling, we may have to.

Slayer and Assassin is worded in such way for some reason.

"X attacks against the same character"
is not the same as:
"X attacks (attacker chooses defending characters)".

Difference is such that in first case strikes from next attack of the creature cannot be assigned to the other character (and cannot be assigned to more than one character if number of strikes is greather than one, which is possible even for Slayer and Assassin).
In second case such assignation would be allowed.

There are things that proves such interpretation. Some cards says that character must face the attack alone or as though he were one-character-company (Burglary, The Hunt).
I don't see a reason for such remarks if "attack against the same character" would automatically mean "attack against character but not a company".