IMO:
1) - None - see 2)
2) No - (NB: all flights are 'loggable' but these are not 'countable' towards anything - you could 'log' them as 'cover' or 'safety pilot'). Now, flight pay and allowances is a diffrent aminal
3) Not legally, but if you miss the TC's big mistake
(wash my mouth out
) or do not pick up a significant problem from where you sit, your company may not think much of you! - 'cos that's why you are there.......................... and of course to take the seat if considered necessary - then they are 'countable'!