By plane perhaps. You can take low fares flight like AirAsia

We caught the train there and back from Marina Bay Sands, they’re cheap, efficient, and plenty of them.

We went from the city centre but it looks like it would take about 30 mins by taxi from Changhi. Not sure how realistic it would be as you would have to go through immigration going out and back in through security so you would only limited time. You would be better going to River Wonders next door to the zoo which is smaller ( but it does have the pandas)

I wasnt aware it was considered wheelchair unfriendly? Grabbing the map and looking at it to remind myself. THe zoo is built with slopes wherever possible. Some areas are steep though - the Baboon area, and up and down to Reptopia are places where there are some steep sloping and the same for some access points to the Primate kingdom but is a lower steepness access point for those areas (though the less steep one for the baboons still has somewhat of a slope. With someone assisting with the chair Id say they would still be completely accessible on the steeper slope access routes although those points would be more of a challenge when self pushing. The trams either have some wheelchair slots, the ability to help you into a seat and transport the chair (though I wouldnt use the tram to get around myself even able bodied). In Fragile Forest you wouldnt be able to get up onto the feeding platform |(steps) but you would be able to go around the exhibit. Elephants of Asia has steps to seating but has a dedicated wheelchair seating area, so does the Shaw Foundation Amphitheatre. All the marked toilets include a disabled toilet Given those I have seen wheeling round or using the rentable mobility scooter Id say it appears fairly well adapted - though I dont have the full perspective a wheelchair user would have.

The ticket sale office is a room next to the entry gates, this is in the undercover area where the food court is. You can pre pay to save time depending when you go. If on hols, midweek is a good time, far less crowds. You can take the shuttle bus from Khatib Station, Bus is located pretty much at the last bit of undercover awning outside station. Enjoy

There is a digital map you can download on entry.

Rain doesnt get a refund on regular tickets. I would assume the same holds for SRVs. I have seen a number of youtubers visit during rain and they never mention getting refunds but I cannot cite personal experience as Im just a regular visitor to Singapore not a resident. Try it and report the result would be my other answer.

Around $35 US dollar. Including night safari. Very hot and humid make sure just wear light clothes especially walking for hours. Buy your ticket online coz maybe too crowded, to make sure and to avoid the long line. Enjoy and have a fun trip.

Taxis in Singapore are safe, reliable and reasonable in pricing. You may take a shuttle bus from Khatib MRT station directly to the zoo or book the Mandai express if you are going to the zoo on weekends and public holidays. However my suggestion is to take a taxi back after a whole day of walking and exploring. Tickets to both the Parks can be purchased in a combo set from the zoo website. You get a 20% discount if your purchase it online. Enjoy your trip!