I don’t think so - there were lines for people to queue if there was a wait - but often there is no wait time

Cannot

It is possible, but you’ll have to be a bit quick about it. Night Safari is only in the night so you have the whole day for Singapore Zoo and River Wonders, but after going to 2 large parks I don’t think you’ll be able to fully enjoy the night safari. I would recommend doing only Night Safari and one of the other 2 in one day.

Depends on how much time you want to spend at each of them. If you are very interested in animals, then you are likely to spend a lot of time at the Singapore zoo, on which case it would be tough to accommodate River wonders. However, if you want to spend only a bit of time at the zoo just to see a few animals, then you could do both in one day. Night safari is in the night anyway, and not that tiring since you don’t have to walk, so it can be easily accommodated. I would also suggest that you try and visit the Jurong bird park. It’s a great park, and you can easily combine it with river wonders and night safari because it is smaller than the zoo.

Yes

There is no minimal age, however prudent to avoid infants!

Too much for 1 day with kids. At least 1 and a half days for all 3