Running into a bug with inline comments, generally LGTM, a few comments.

• Short comment on the difference between obedit_ctx_mode & obedit_mode would help.
• Would prefix object's with ob_, find Object *armature doesn't read well.
• While OBWEIGHTPAINT_FROM_OBACT is OK, not sure it warrents it's own macro for a single use? (no strong opinion on this if you think it's useful elsewhere).

Tested with T66949: Difficult to select bones for weight paint with multiple armature modifiers, this still requires two clicks when switching between pose objects which are obscured by the geometry.

Although this could even be an advantage (that the active pose gets favored and you can't easily switch to another pose unless it's directly under the cursor).

Just noting it as an issue that users might notice in the future, don't think it's a blocking issue.

Seems fine here too. Looks Good To Me!