It says, "if you are the leader, you can activate a Defensive Codex ability once per battle at a cost of -1 Cohesion." Then, next sentence, "also, you can activate (and benefit from) Defensive Squad Modes of other chapters."

The connection between these sentences is not clear. Can the second ability be used only if you are the leader, or always?

It's not clear. I would say the second line applies to leader only, but you should really send a query to 'Rules Questions' with the link at the bottom of the page.

Presumably you do not have universal access to all Chapter Squad Mode Abilities, only the ones available that mission. These would be dependent on who is the current Squad Leader.

Unless I missed something and the Battle-Brothers can activate their own Chapter SMA's at any time regardless of who is the Squad Leader.

-K

Clarification from emails and errata has idicated the following.

A leader or any kill-team member may activate any codex squad mode they have access due to their oath.

A leader or any kill-team member may activate his own chapter's squad modes (he knows them all).

Any kill-team member may only activate a single squad mode ability a turn.

Any kill-team member may sustain or benefit from only one passive squad mode ability at a time.
- If a kill-team member is sustaining a passive ability and wants to benefit from another passive, he must first
cancel the ability he is sustaining.

Any kill-team member may only benefit from a single squad mode action (charging/attacking/moving out of initiative) a turn.

Only members of the same chapter benefit from chapter squad modes, unless special rules or talents allow otherwise (these rules or talents will say so, it will not be hinted at).

Right, but DG gives you the special ability to use other people's squad modes. It's just not clear if you have to be the leader to do so.

Ok, read the Deed. (Would have saved time If I knew we were talking about a deed, I thought you were talking about a squad mode or something. Please be specific in the future.)

Your character must be the leader to benefit from the discounted cohesion cost. But, it does not appear your character has to be the leader to benefit from another kill-team member's defensive stance, although even if another kill-team member activates the ability if your character benefits from it the cohesion cost is increased.