To Question 1 (River Safari): Even if you intend to take a lot of pictures, 2,5 hours would be more than sufficent. This time schedule would also include the animal show, which I unfortunately missed (and so the crab-eating racoon).
To Question 2 (Night Safari): Yes, bigger animals like Asian Elephants, One-horned Rhinos, Gaurs, Bantengs as well as Dholes (and others) can only be seen by tram.
For taking pictures you have to keep in mind, that using the flash is prohibited in the whole park (Although I have seen some guys that have risk to use it). The exhibits are partially illuminated by artifical "moon light". But this is normally not enough. Use at least 3200 ISO (and even then it can't be sufficent for a good picture from an animal that is moving).
To question 3: I would recommend your plan, doing Zoo and Night Safari at the same day. It can be exhausting (drink a lot, and - very important - don't forget the insect repellant), but you can safe two times the cost for a cab (From downtown Singapore to the zoo it is about 22 to 25 Singapore Dollars one way - higher at night). If you have been succesful by taking picture at your first day in the zoo, then go to the brand new S.E.A. Aquarium (Marine Life Park) on Sentosa Island. It is brilliant and they have 2 species of manta rays on display!