We had a situation where a group of sentinels w/ a commissar had only one survivor after an engage action, the fearless commissar. Is the formation considered to be broken even though what is remaining of the unit is fearless?

Since the unit only can have one blast marker, would his Leader ability remove that only blast marker causing an auto-rally in the end phase?

Also, how does Commander work exactly? If the other units used in the engage have already had an action (march, etc.) can they participate? What about if the participating frms haven't had an action yet, can they take one later, or is the engage considered their action for the phase?

He is indeed broken. Fearless does not affect the number of BM it takes to break a formation. Even if there was no commissar, a one-unit formation would shed all BM if it rallies - you halve the number of BM and round down.
The commander ability can only be used on formations that have not activated yet, within 5cm of the commander's formation I believe

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I'll try and answer.
1) the formation is considered broken. fearless formations can still be broken, they just dont take extra hits.

2) No auto-rally. If the rally roll is successful then the 1 sentinel formation would come back with no blast markers. Leader only works after the successful rally roll.

3) No, formations that have activated already cannot participate in a combined assault using leader.

4) The engage counts as their action for the turn.

Hopefully that is a) useful and b) right.

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Leader ability only comes into effect once you have successfully passed a rally roll, so no auto-rally.

A Commander can call upto 3 formations, including his own and that have not already activated, into a single engagement. If the initiative test is passed then this is the action for all the formations involved. If it is failed, then only the commanders formation is counted as having activated. Initiative is rolled off the commander's formation with blast markers from other called in formations being taken into account.

Thanks! Pretty clear now.