I do this journey regularly. My mum's 85, arrived on the 18.20 from AMS, with luggage, and we easily caught the 18.59 train to Malmö. It might take you a bit longer if you don't know where you're going, but an hour should be plenty.
When you exit customs into the arrivals area, the exit to trains is to the left. If I remember rightly, you need platform 1, but check the signposts.
The Skånetrafiken app is best. You can buy your ticket in advance and not activate it straight away - you can activate it once you're through the airport and have found the platform. The trains run several times an hour. The platform announcement boards are not always very helpful, so the app is also the best source for when the next train is leaving. If in doubt, ask someone else on the platform.
I haven't been outside Malmö Central at night (I usually just travel through or else I'm there during daytime), but based on my general sense of the area I would be comfortable walking a few blocks from the station. If you're travelling soon, it may well still be light when you arrive.