I think I've made it fairly clear that I don't regard hit point loss of the sort that might occur in combat, or as a result of a skill challenge, as "an impediment to the ability to impact on the shared fiction" in the relevant sense. I am talking about things like being deprived of class features or XP on an ongoing or permanent basis. As I said, for the same reason I don't think I have ever had an AD&D undead drain a level from a PC.
So, again, you are not opposed to the removal of abilities in principal, only on the degree of the impairment (its duration, anyway - I had also thought breadth of abilities removed, but that's not clear from the above). So it's OK to deny access to one or more class features for some period of time, but not a period that makes it "ongoing", however long that may be.
Given that, I stand by my previous assessment that we are no longer addressing an absolute principal, but a question of the degree to which the player's ability to impact the fiction can be impaired.
I don't know quite what you mean by "the usual action resolution mechanics", but in this particular episode of play the skill challenge mechanics were being used.Those are one component of the usual action resolution mechanics.
See my post above for an idea of what I mean. I mean not "By fiat, your familiar is not available", but an actual rule you can cite by which the familiar is removed as a consequence of an action taken by the player in the skill challenge.
I also find your conclusion you have been "fairly clear" stands in contrast to the number of posts indicating a lack of clarity for various readers, but I guess you are entitled to your opinion of your clarity.