1. On ba.com, at the final stage when you click ‘pay’. Via the call centre, as soon as the agent holds the inventory. This is one occasion where it can be beneficial to use a contact centre, particularly if you get a competent agent. This is especially true in disruption, where a proactive one will hold a seat while you discuss, rather than choosing it after, as they can disappear quickly.
2. Online, it isn’t held. Via the call centre (or TA), as soon as the agent exits your booking.
3. Yes, subject to the (usual) constant re-evaluation of inventory control. Note the caveat over redemption bookings in that if the total number of booked redemption passengers is lower than 2+4 and it is 45 days or more before the flight, the redemption space will release back to the public. This is thanks to the very public guarantee they made on redemption availability. Only if either of these is false might the seat not be re-released.