Without getting involved in this little spat, I'll answer the succinct question asked in the OP title:
"Detraction - When are we guilty?"
We are guilty of the sin of Detraction -- a sin of the tongue against Charity -- when we
1. Reveal a hidden/private sin of another living person,
2. The sin in question being TRUE, (or else it's slander or libel)
3. Without sufficient cause (e.g., the public good)
Note this doesn't apply to criticizing another's public posts -- or any public actions. Anything in "the public domain" can be discussed. This includes anything the public has a right to know about, or needs to know about. The background, positions, and beliefs of Politicians for example. Also, the Church is a public organization, and a Priest is a public man. So anything touching on a priest's formation, ordination, or "resume" (curriculum vitae) is certainly public domain. Detraction doesn't apply to anyone who isn't "valid matter" as a target for the virtue of Charity, such as a Corporation or an Organization.
No individual is beyond criticism, especially on a discussion board. Most of all anyone famous or pseudo-famous, including anyone holding anything remotely resembling a leadership position. Forum owners and moderators, for example.