Two Questions:

1. Can Guardian Pillar's 'ability' be blocked by a disrupt (by Performance Artist, Power Drain, etc.)?

2. What happens if I pull Stygian Eye or Infernal Obsession from the discard pile, using Jamburg, and attach it to Guardian Pillar, while it is a character?

1. No. Guardian Pillar's effect is a passive effect, and passives cannot be cancelled.

2. You can attach the Stygian Eye or the Infernal Obsession, since it is a character at that time, so the "Attach to X" requirement is fulfilled. The attachments are not discarded after the Guardian Pillar mutates back into a support again, since play restrictions are only checked while playing the card and not later on:

An attachment only checks the
requirements for attaching it when
the card enters play.

Where I'm uncertain is, if you may keep control of the Guardian Pillar after it mutates back into a support. I can't answer that, but because both attachments are formulated in the way "take/gain control of the character ", I'd guess the effects are not valid anymore. (But since the Obsession stays on the Pillar, your opponent usually won't commit it to a story again, because you would regain control of it again. So at least it would force the Pillar to be unusable.)

FAQ quote:

When a card changes type (character, support, event, or story), any cards that are attached to it are immediately discarded from play.

qed.

Yay, got me there :-) Problem solved.

Hm, where in the FAQ did you find this? I searched for it several minutes now, I must be stupid, I absolutely can't find it…

Lol, it was removed…

it was in section (1.4)Attaching Cards Through Card Effects, i had an earlier version of FAQ in HTML, it seems to be removed in current FAQ…

It was the last sentence, after "The card retains its faction identity and printed cost."
There is also "For the purpose of all other card interactions (excluding interaction with the card that initially caused the attached condition)," added before "the card loses all of its original characteristics (such as skill value, icons, and type)."
And my older version mentioned Tear gas as an example. (and this point was 1.3 not 1.4 back then)

Is there a FAQ archive to check when was it removed? This section was changed a bit, I wonder is it intentional change, or just one sentence was lost when changing this section.

If its intentional rule change i agree with "it is still attached, but support is not character, if GP uses its ability it will be controlled by attachment owner"

Removed, hehe ;-) Now I'm relieved.

Ok, the question really is if this omission is intended or accidental. In doubt, I'd rather guess it is intended. (It looks more like a deletion to me). But Dark Initiate, if you want to be sure, forward the Pillar+Stygian question to FFG/Damon, he'll answer it and you'll have confidence.

This card is nuts. I wish they'd errata it so that it wasn't a passive, because it doesn't make sense. Are there any other examples in the card pool of triggerable passive abilities?

Yes, non-passive effects are much easier to understand. Everyone can just look into the timing structure of an Action, and immediately knows when to resolve Disrupts, Responses etc… With passives, this is sometimes very difficult.

The Pillar's effect could be worded as an Action easily (likewise to Dirk Sharpe, e.g.). The difference is, Actions can be cancelled, but passives cannot. I guess that's one reason for the Pillar to be written as it is. There is another passive that is similar, although I had to dig a little for it: Sweet Old Lady (Opponents must pay 2 in order to commit characters to a story where Sweet Old Lady is committed alone.) Her passive also follows the "pay a cost to commit to story" pattern. But I guess that information isn't very useful ;-)

If you want to know how it works… In contrast to e.g. Dirk Sharpe (which is commited not with all other characters of you, but either before or after), Guardian Pillar is committed concurrently with all other characters of you. That's another reason for his text being a passive - usually, supports can't commit to stories, but this passive "bends" the rules so that this card can also commit to the story alongside other regular characters.

Sure, I get that passives cannot be cancelled, but then they could just do something like Dunwich Horror that has the text: "This ability cannot be canceled or disrupted."

Pillar can only be committedd during normal commit slot, it has to be passive. Action is not trigerable during this slot and could allow tis card to be commited in other timing. Forced response would force to commit him, responce would be triggered after characters have commited making pillar commit alone. Disrupt could be used, but disrupts arent really that much better to understand.