Yes buying singles is indeed sensible for maximum flexibility, but the implication being that in some flight combinations that gets to be more expensive due the way that carrier surcharges are calculated, but cheaper on other routes. Also if you end up cancelling the whole trip you could end up paying the £35 fee twice over rather than once.

But yes, subject to availability, and subject to staying in the same zone, you can change the return flight after flying the outbound, so long as it is done more than 24 hours from flying and subject to availability. You just can't get a refund unless there is an involuntary component to it, such as a cancelled flight.

For where the companion passenger has missed the outbound, by rights BA can cancel the return booking for both passengers - they have never done that though. However they typically will cancel the return flight of just the missed passenger, if the outbound is not taken as advertised. If there are visa issues to be resolved, that frankly is a reason not to use restrictive instruments, save them for a better occasion.