Protection From Falling

Does the guidance recommending at least 2 m headroom over a stair apply to alternating tread stairs?

Yes it does.

Some types of guarding leave a triangular opening between the bottom rail of the guarding, the tread and the riser. Is this acceptable?

The Approved Document does not give guidance for this type of guarding. Subject to the approval of the building control body, this type of guarding will usually be acceptable in buildings other than dwellings, providing the bottom of the guarding is not more than 50 mm above the pitch line (measured perpendicular to the pitch line). 

Approved Document M (2004) also gives guidance on stair and ramp design. Where there appears to be a conflict between the guidance in Approved Document K and Approved Document M which takes precedence?

The guidance in Approved Document M deals with ensuring ease of access to and around buildings, which is different to the minimum standards necessary to secure the safety of people using stair and ramp design.  For more detailed information see the FAQs for Part M.