If informed well beforehand like 2 months ago then the couple will release the seat to another guest. As for angpow as blessing, depends on your relationship with the couple, it’s good to give if u are close to them. :)
Normally 1-2months before couple will confirm no of pax with the organiser/hotel/banquet again So it's fine not to give any pow, of course will be good to give
You’ve informed them well in advance, so there’s no seat to cover:) Just a blessing will do, if you’d like.
It is totally up to what you are comfortable with. i would still suggest that a small gift would be nice
I think it is okay not to give, if small token depends on your relationship with the couple
A small ang pow would still be nice(:
A blessing is nice